Wow, that's tough one but I love these top 12 seasons idea, so I'll try to do it right
Moses best seasons:
1983
1982
1981
1985
1979
1980
1987
1984
Barkley best seasons:
1990
1989
1993
1991
1988
1992
1995
1994
1996
My list:
1. 1983 Moses - by far the best defensive season available in this comparison and very strong offensive performance as well. It was his best defensive season, as he put more effort on that end than ever before. My eye-test is backed up by +/- we have. All-time great postseason run is just icing the cake.
2. 1982 Moses - despite massive offensive load, I have seen a clear defensive improvement from Moses in games we have from this season compared to 1981. This year was the year when Moses made a big jump on both ends of the floor. He improved his jumpshot and faceup game, which made him more versatile offensive player. That allowed him to do this huge carryjob in RS, which I often compare to what Jokic did this season (not saying he's as good).
3. 1990 Barkley - one of the most impressive offensive seasons ever from a big. This was Barkley at his absolute peak, when he was still marvelous athlete but already had refined post game and ball-handling ability. His performance against Chicago in the playoffs was very impressive, more so than 1993 finals in my opinion. Higher motor than older Barkley as well.
4. 1989 Barkley - not too far from his 1990 season, it was just a little less complete and postseason performance wasn't nearly as good.
5. 1993 Barkley - quite close between 1981 Moses and 1993 Barkley, but I decided to go with better RS from Chuck. His postseason run was quite good, though overall a bit worse than Malone's, but I think he did a bit better in the finals than Moses (though he had more help). Moses was notably better defensively, but he still had some lapses on that end and I'm not sure how big of his impact was on that end.
6. 1981 Moses - this one is clearly Moses for me. Barkley had notably bad defensive season in 1991 and I don't think his RS overall was clearly more impressive.
7. 1985 Moses - very good RS performance from Moses, one of the greatest foul drawing season ever from a star and very solid defensive impact as well. Wasn't too hot in the playoffs (to say the least), but this 1985 Philly team was much worse than it looks at first glance and Moses was the only high impact player from their starting 5. The fit with rookie Barkley was also terrible.
8. 1979 Moses - not as good defensively as later versions, but insane motor and likely his overall best rebounding season. Anchored number 1 offense in the league, though he had considerable amount of talent on that end to be fair. Despite questions about his defensive effort, I still think he was better than 1991 Barkley on that end.
9. 1980 Moses - similar to 1979, slightly worse RS but solid postseason performance (despite Rockets being outmatched by Boston).
10. 1991 Barkley - don't see any other Moses season approaching that level of offensive dominance. Barkley was still close to his peak offense here, it's just that his defense wasn't there.
11. 1988 Barkley - not a huge fan of younger version of Barkley (huge gambler on defense, turnover prone), but can't deny that it was very impressive season from him.
12. 1992 Barkley - missed the playoffs, but I don't think any post-1985 Moses has a case over that version of Barkley and older Barkley wasn't nearly as resilient offensively due to diminished athleticism.
I have these two very close on my all-time list and I think this list shows why. Moses peaked higher in my opinion, but Barkley had slightly more consistent prime. At the same time, Moses catches up him with his post prime career (I view all of 1988-91 seasons as clear positives).