Yes, the C's have one of the best bench units in the league.
And what's wild is the Celtics' bench scoring efficiency is pretty much the same/better than their starters. Defensive efficiency is better, too, but that might be a product of facing opposing bench units.
Some other candidates:
1) Golden State: Like others have mentioned.
2) Toronto: They bring TRoss, James Johnson, Louis Williams, and Psycho T among others.
3) Houston: Josh Smith and Corey Brewer could be defensive-oriented starters. Terry can still shoot.
4) Suns: Gerald Green and Isaiah Thomas bring a ton of scoring. The other Morris and the other Plumlee are competent.
5) Lakers: I'm a big fan of the Lakers' bench. Lin, Boozer, and Swaggy P often outperform the starters.
6) Spurs: Lots of competent NBA players and Manu. They've found a gold mine in building their roster with foreign players.