Of the 3 existing bigs we have which works better
Monroe sully
Monroe Kelly
Zeller Monroe
I think Zeller Monroe works best but I think Monroe sully are the two best players and Monroe KO has the potential to be crazy on offense at least
I think Olynyk, because Olynyk is a triple-threat offensive player - he can shoot the three, he can put it on the floor, he can pass, and he has excellent basketball IQ.
That's exactly the type of guy you DON'T want to leave open on a double-team attempt because if Monroe passes out of the double team to Olynyk and the defense reacts and runs at KO, it's very to predict what he's going to do. If they close out hard or send a bigger defender running at him, then he can put he ball on the floor and run straight past the defense. If they close out soft or send a small guy at him, he can make use of his 7 foot frame and shoot right over the defense. Even if they do manage to close out effectively, he's got outstanding court vision and will find the open guy more often then not.
Tough, tough offensive matchup for the opponent.
My second choice would be Bass because even though he doesn't have three point range, he's a career 46% shooter from 10 feet to the 3 point line. If you leave him open he'll hit the jumper 8 times out of 10. If you challenge him, he's very good at shooting that shot over the defense. If you run at him hard, then (like Olynyk) he can put the ball on the floor and make a hard drive to the basket.
He's also got a freakishly long wingspan (relative to his height) which, when combined with his good lateral mobility, allows him to defend anything from small forwards to centers. This helps to counter Monroe's lack of defensive mobility to a degree. Also while he's not much of a rim protector, Bass dead lead the Celtics in blocks this year (1 per game).
Neither player is much of a rim protector, but I think either combo would work.
I think Zeller and Sully would both be really bad pairings. Zeller because he's the worst defensive big on this team (his PER allowed of around 22 is absolutely horrendous) and Sully because he's not scary enough from three or midrange to scare teams out of doubling Monroe...and if they do leave Monroe and run at Sully, he doesn't have the ability to put it on the floor like Bass and Olynyk - he's more likely to force a contested three, or hold the ball and look for the pass. Of the opponent can stay on Monroe and leave Sully open, since there's a 75% chance(as a ~25% 3Pt shooter) that he'll miss the shot anyway.
Bass is a lights-out shooter from midrange and is a versatile defensive player - these help to make up for Monroe's lack of foot speed and shooting range. On the other hand Monroe is a dominant post player and rebounder as well as a very good passer - all areas Bass is weak.
I think those two guys compliment each other pretty well.