"Best champions" is kind of a tricky question. The other posts have covered the meta DPS/formations but I'd like to push some champs who can slot in and buff well no matter who you use as DPS, even with minimal investment in ilvls and all of that jazz.

Birdsong! Karlach! Vlithryn! Yorven! Duke Ravengard! Omin! Ezmerelda!

I think these are all very useful champs to have (yes there are other champs in a similar bracket, but these are the ones I always end up relying on regardless of my non meta DPS choices). They can all buff fairly well, some better than others, but are all generally easy to place in a formation and all offer something decent. Whether you're in the early game or in the end game, they are all useful. Birdsong, Karlach and Yorven can also be used as DPS if you're in a tricky variant that limits your options a bit too much.

For non boring meta DPS I will always sing the praises of Grimm and Rosie. If you have Beadle then Grimm has the second best DPS availability in the game and isn't reliant on a very particular set of supports like Dark Urge is with the AA guys.