Completely disagree with Wall getting voted in before Horford. People are really underestimating what Horford meant to Boston this past year. And I believe the impact stats really don't tell the whole story here. He ranked 19th in RPM and then 41st in RAPM (non prior-informed, mind you), with Brown and Tatum coming out ahead of him in the latter. And this is largely because Horford missed 10 games, during which Tatum and Brown saw a decent chunk of their minutes together without Big Al and the team performed well. On the entire season including playoffs, Tatum and Brown only played 232 minutes together with Horford 
off. 
Horford, Tatum, Brown ON: +9.4
Tatum, Brown ON; Horford OFF: +8.7
And so this period of time that Horford didn't play essentially leads the model to believe that Tatum and Brown are more responsible for Boston's success than Horford. I mean Boston did manage to go 8-2 without him. But when you consider that in the 10 games Al missed, 7 of them were at home and then 5 of their 8 wins came against: 
- Brooklyn at home 
- Brooklyn
- Chicago at home
- Los Angeles Lakers at home
- Charlotte at home
- Nuggets at home (who I put in this category because they're 
significantly worse on the road - 15-26 on the road vs. 31-10 at home - and as shown in the graphic in this article 
https://deadspin.com/which-teams-in-each-sport-have-the-biggest-home-field-a-1828880402, they have easily the largest homecourt advantage of any Big 4 sports team)
They lost to Indiana at home. They lost to Washington at home. They beat the barely above .500 Clippers on the road. And then they had a super impressive win against a red hot Utah team in Utah.
My problem here is that this 10 game sample size without Horford carries too much weight when you consider that A) the competition was very weak as a whole, and B) Brad Stevens might very well be the GOAT coach at maximizing talent during the regular season and getting guys to play above their heads. I mentioned this before in discussing Kyrie that this is one reason why I was willing to give him a little more credit despite his meh impact stats, as Stevens can keep the train going no matter who's out (though to be fair, Kyrie's RPM/RAPM were nothing special in Cleveland either). But I don't think it's sustainable over the long run and I 
definitely don't think it's something that would carry over into the playoffs when you consider that teams actually extensively game plan for their opponents. And that all goes along with my final point C) that it's not a great sample size.
It reminds me of 2008 when the Celtics went 
9-2 without Kevin Garnett during the season. But how could this be? Garnett was most likely the most valuable player on the season. Wasn't he the guy who was anchoring the historic Celtics defense and mostly responsible for Boston's turnaround from 24 wins to 66 wins? Yes, that's all still true. But Boston played some weak teams and it's a relatively small sample size in which great teams can still keep the ship afloat even while missing a very good player. But that doesn't take away from what Garnett was doing the 87% of that regular season he was present in which he was clearly the team's best player - something that carried on to the postseason, as well. And then what happened in 2009 when Garnett missed the playoffs? The 62 win Celtics barely squeaked past a 41 win Bulls team in 7 games, many of which went down to the wire, and then lost in the next round to the Magic. After winning the title the prior year. Then he returned and the next year with Garnett they beat the 61 win Cavs, then the Magic who had beat them the year before, and then came within a couple minutes of another title losing Game 7 of the Finals 83-79 despite Boston's starting center Perkins not playing in G7. So he carried an enormous impact, which couldn't be doubted, yet in that 11 game sample from 2008 without KG, many people would use that against him to argue that he's just not that important. And I think it's very similar to the case of Horford last season.
Here's what I had to say about Horford during the playoffs last year:
Boston has the #1 defense in the league and the #18 offense in the league. They win games with their gritty defense and Horford is the backbone of that D with his versatility and deterrence at the rim. And this is nothing new for Horford. In fact if we look at the past 4 seasons, here's how the teams Horford has been on have ranked on defense:
2014-15: #6
2015-16: #2
2016-17: #13
2017-18: #1
One outlier year in 2016-17 and why is that? Oh yeah, because the Celtics had a tiny backcourt of Isaiah Thomas - the worst defender in the league - and Avery Bradley, who contrary to belief isn't actually very good of a defender, and Detroit and now LAC are finding that out. His on ball quickness and peskiness look good to the eye, but he's a poor team defender who's not all that active in the passing lanes and doesn't do much to deter anyone with help defense. The impact metrics haven't painted a pretty picture of him on defense. So the fact that Horford was able to anchor the team while being held down by that backcourt is extremely impressive. 
What's also impressive is, look at how his teams have fared the last few years:
2014-15: 60 wins, Conference Finals (loss to Cavs)
2015-16: 48 wins, Conference Semi Finals (loss to Cavs)
2016-17: 53 wins, Conference Finals (loss to Cavs)
2017-18: 55 wins, Up 2-0 in Conference Semi Finals...while missing the 2nd and 3rd best players
The guy is just a winning player who's been a part of some fantastic teams and arguably the leader of all those teams. So why do people continue to underrate him? Is it because he put up 13 ppg, 7.5 rpg, and 5 apg in the regular season? Those are actually phenomenal numbers especially when you consider half his impact comes from the defensive side of the ball. He's an amazing playmaker who scores on an efficient 57.5% TS and shot 43% (!) from three on 3 attempts per game while playing 57% of his minutes at center, so his floor spacing is really valuable being that the majority of the time he forces the opponent's 5 out to the 3 point line. Not to mention he was playing 31.5 mpg so per 36, he's at 15/8.5/5.5. I don't understand where there's this idea that if you're not a 20+ ppg scorer you're automatically excluded from a top 25 player list. Whereas guys who give you that 25 ppg but provide little else are seen in a much better light. 
And then Horford raises his level of play in the postseason. This season he's averaging 18.5 ppg, 9.5 rpg, and 3.5 apg while scoring on an absurd 69.7% TS. For reference, in last year's playoffs he scored on 66.8% TS. 
But people still underappreciate the guy, attribute all of Boston's success in spite of the injuries to Stevens, and say that the Celtics are missing their 2 best players (aka 2 best scorers) perhaps as a slight way of hyping up Boston a little and what they can achieve when healthy. And don't get me wrong they're going to be incredibly scary when they all together, but I just disagree with the notion that Horford isn't their best player.
Hayward misses the entire season. 16th ranked player in the NBA according to our poll here Kyrie misses 22 games and the entire postseason. And so Boston relied on their #1 ranked defense anchored by Al Horford - who has a track record of anchoring extremely good defenses - in order to win 55 games, beat a Philly team that went 
22 and 5 after the All Star Break in 5 games, and just 
barely miss out on the Finals after losing Game 7, making for the 4th consecutive year that Horford's run was once again cut short by LeBron James. That's 3 times Horford's team made the ECF and was only separated from the Finals by playoff LeBron. And I don't think him failing to get past LeBron is something that should be held against him so much when we're talking about a guy who might not even be voted in as a top 25 player in the league not managing to win against the consensus number one player in the league, who's also had a better supporting cast 2 or 3 of those seasons.
And again, Horford was the 
best player in a series featuring a guy who ranked 11th best in the NBA in this poll and another who ranked 22nd best. He averaged 16 ppg on 63% TS, along with 8.5 rpg, 3.5 apg, and sensational versatile defense from the C position that allowed Boston to switch virtually anything. Also iggy, I've gotta disagree with you on docking him because he's 32 as of a few months ago. For bigs that aren't very athleticism reliant, we really shouldn't expect a sudden, steep dropoff and he reminds me of Duncan in a lot of ways (obviously much worse) and I really think his game will age very well. His production certainly doesn't seem to be trending towards a serious decline and he just came off the best season of his career and it's not like his season was helped by any anomalies in what he did that we shouldn't bank on moving forward. He's as good as he is because 1. he's a phenomenal, extremely intelligent defender, 2. he brings point-center ability to the table, and 3. he spaces the floor well. The only one of those that you could be wary of is his floor spacing as he shot 43% from three which likely won't happen next year. But I don't think that's super important that he shoots above say 40%. As long as he shoots reasonably well, which I think he'll continue to do. This is a guy who attempted between 0.1 and 0.5 threes a game his first 8 seasons. Then these past three years he's attempted between 3.1 and 3.6 as he's focused on making it a deadly aspect of his game and he's shot 34.4%, 35.5%, and 42.9% from that range. As long as he's hitting somewhere in that range of values, he's spacing the floor very well. And then looking at his playmaking, he averaged between 2.5 and 3.5 apg in Atlanta and that's jumped to 5.0 apg in Boston. So that's yet another part of his game he's taken to another level as he's gotten older. And just looking at his production level over his career:


He's posting a better RS BPM in Boston than Atlanta despite being over the dreaded 30 mark and nearly double the PO BPM in comparison.
Horford is really a mini-Draymond in a lot of ways. He's not as good of a defender or passer (better scorer though) and overall he's obviously not as good of a player, but he does a lot of the very same things Green does, which tend to be undervalued by the general public. Both are crucial to their team's success yet people will always focus on their PPG.