The answer to your question is "Anyone that can successfully fight MK and not get gimped". Let's see, that includes hmm........ MK, Kirby, Snake, Pit, Wario. Now there are other characters, but they are less efficient. My personal favorite team mates include Pit and Wario. Pit because on Final Destination (or any stage) Diddy and Pit together can setup a bulwark that canNOT be broken. A wall of arrows, peanuts, bananas, and mirror shields stop and deflect anything. So those two can sit back and camp the entire time with success. Wario is good on Diddy's team because of his power and air mobility. With Wario's air speed and Diddy's ground speed they can set up these moving zones where it isn't safe to jump or walk along the ground. I like DDD and Diddy, but only Diddy can stop metaknight from taking over the stage. If there are 2 metaknights, DDD will subcumb to all that speed and Diddy will get hit just trying to save him. Double Diddy is "ok" BUT only if they both are good at fighting with out bananas. The recommended strategy for them , have one spam while the other fights. This team is mainly for fun though, cause they both have the same weaknesses. Another personal favorite is Snake. His killing power compliments your combo ability and if he knows how to use bananas and you know how to use grenades then winning is cake.
http://www.youtube.com/watch?v=JHUGm5u2lBg
http://www.youtube.com/watch?v=PYdbAjgGCMU
Those 2 clips show decent wario / diddy team play.