Marth is pretty good with most characters. I love Falco because in a neutral position, he can high laser and you can stay low, absolutely demolishing the other teams options. He can also lock down people while you are chaingrabbing/doing a monster combo. Marth is really good at protecting him too, so I think it works nice. Peach is great because falling fair puts them back into dsmash. You can have sequences with Peach/Marth where you do 160 damage in like 5 secs lol. Jiggs, Fox, and Falcon are great because they have great punishes off your grabs (rest, usmash, knee), though Foxes need to be babysat a little. Gimping 8 stocks with a Sheik is maaad fun.
I dunno, its all about how you like to play. I'm generally stupid aggressive so Peach and Falcon work great with me. A Falcon who just waits for you to create an opening and starts a zero-to-death is awesome to play with.
If I had to pick three, I'd say Fox/Falcon/Peach.