When you take out human error (like screwing up chain grabs, etc), TOP LEVEL PLAY, I think #1 is undisputed, and #2-7 are very interchangable because they're top characters of similar potential. I think it's:
1. MK
2. Ice Climbers
3. Falco/Diddy
4. Falco/Diddy
5. Snake
6. Marth
7. Wario
I've talked with V1CE offline a lot about this too.
Also lots of top players believe that ICs are 2nd best, including Anti, me, Vex, Orion, and others.
So Diddy has no chance of being 2nd because he loses to Peach? Falco has no chance at being 2nd because he loses to Pikachu? That logic has lots of flaws :/. Also, if ICs lose to ROB, it's VERY slight. I think it's either even or they win, though. Certainly not a bad matchup, regardless.