I would like to have threads for most of the Top 30-ish guys like a mini-project or something, so hopefully there is some response to this. And I didn't want to start with more Jordan/Lebron so I figured I'd try two guys with no rings.
This is relative: If a team wins 57 games and no one else wins more than 52, they're probably a contender. If a team wins 57 games and 3 teams win 65+ and they have to beat all of them on the road, they're probably not a contender.
This is not a supporting cast question: If Bill Russell plus 4 guys from the Y win the title, that's a contender. If Charles Barkley and 4 guys from the Y win 30 games, that's not a contender. Of course, if it's Barkley and 4 all-stars and they only win 30 games, you could still say they were a contender and just underperformed.
This is dynamic: It's really a "Did a team have a chance to win it all?" question. If things outside of a team's control help/hurt, take that into account. This includes injuries. The 2000 Spurs might have been contenders, but not once Tim Duncan missed the playoffs. However, if it's new information in a team's control, like if they're the 8th seed and upset the 1st seed, that's just a great series by a non-contender, not the birth of a new contender. The 2007 Cavs aren't necessarily contenders even while making the Finals, the 2007 Mavs are even though they lost in the 1st round.
Optional (but not optional): Apply weights to contending seasons. This isn't championship odds so the numbers aren't supposed to add up to 1.0 for each season. I would say something like below, but choose whatever you want:
1.0 - Favorite / Co-Favorite ('85 Lakers and Celtics?)
0.75 - 2nd best team but there is a strong favorite ('97 Jazz?) / Top 3-4 team in wide open field
0.5 - 2nd best team but there is a dominant favorite ('96 Sonics / '17 Cavs?) / 3rd best team behind co-favorites / Top 4-6 team in wide open field
0.25 - Fringe contender, lots of things need to go right, but not like "once in NBA history" right (if you're the 5th or maybe even 4th seed, you probably don't qualify most years)
So maybe for Barkley:
1985 - 0.5
1993 - 1.0
1994 - 0.75
1995 - 0.75
1997 - 0.25
Spoiler: