No, Isai and <insert any person here> is easily the best team ever.
QFT
Anyway, I'm back to say that Mario is Mario's absolute worst teammate, because Mario does not play off of Mario's strengths at all. The statement that "Mario is the most balanced character, therefore two Mario's will have no weaknesses mathematically" or something is ridiculous, and contradicted in your own post when you say "(save KOs)".
Mario has 3 weaknesses:
1) Approaches
2) KOs
3) Gay Characters
Mario has 3 strengths:
1) Combos/Juggles
2) Speed and Mobility
3) Edgeguarding
Optional: 4) Mindgamez
Teaming Mario with Mario essentially amplifies your weaknesses tenfold. Your approaches are seriously limited. What are you going to do, approach that sheik with a flying sex kick while your other Mario tries to slide under him? LOL YOU BOTH JUST GOT DSMASHED! Worst team ever. Ok rant done.
In other news, Peach is a good teammate, but again, not alot happening in this world that coincides with what Mario is doing, other than that Peach is great on teams. They are both much more edgeguarders than KOers, which sets you up for defeat vs. a good rescue Fox/Falco/CF based team. Also, both are heavy mindgamers, which means you spend alot of time screwing around and being concerned with spacing and proper turnip/fireball/capeage than really fighting, which I feel is limiting. Also, Mario cant really combo into Peach, save Fthrow/Bthrow/Dthrow to Fair, and that's hard to set up. This team would probably fall to a CF/Jiggs or CF/Marth team, or a Ganon/Ganon team pretty consistantly. Peach is also a good stock tank, and therefor probably better off with Gannon and CF.
I'd like to add Fox to the list of teammates that are good. Fox does all things well, and Mario gives him his space, and doesn't really have to worry about getting interrupted or accidently KOing Mario. Mario grab to Fox Usmash will be a primary killer in this matchup, along with plently of shine/capespikes.