No one knows who's best, yet xD... but here are the current rankings o.o
I went and made a rankings list based on tournament performance. I'm also continually updating it. Basically, +0.5 for placing top8, +3 for placing top4, +5 for winning a tournament. And if you want your tournament to count, give me a top8 listing with character mains.

S Rank will see THE MOST representation in a tournament, by far. Especially toward the top. The A Rank characters appear a lot but see fewer winning placements than S Rank. B Rank is the characters that likely do well regionally but don't appear nationwide. C Rank is interesting... it mostly shows characters that have been doing relatively well in tourneys, though likely only showing up in local ones and thus not seeing as much play as upper ranks have. D Rank is popular characters that haven't really placed well, E Rank is just plain unpopular.
Or, in other words,
S: Amazing
A: Very Good
B: Above Average
C: Average
D: Below Average
E: Bad
Easy-to-Read Version
S: Snake, Meta Knight
A: King Dedede, Marth, Wario, Mr. Game & Watch, ROB
B: Donkey Kong, Wolf, Pikachu, Falco
C: Pit, Olimar, Ike, Fox, Lucario, Kirby, Toon Link, Lucas, Peach, Zelda, Ice Climbers, Sonic, Zero Suit Samus
D: Luigi, Sheik, Diddy Kong, Mario, Bowser, Ness, Captain Falcon, Ganondorf
E: Jigglypuff, Pokémon Trainer, Link, Samus, Yoshi
S Rank
1 Snake (46 top8, 39 top4, 28 wins, 280 pts)
2 Meta Knight (44 top8, 53 top4, 14 wins, 251 pts)
A Rank
3 King Dedede (20 top8, 26 top4, 13 wins, 153 pts)
4 Marth (31 top8, 31 top4, 3 wins, 123.5 pts)
5 Wario (8 top8, 12 top4, 14 wins, 110 pts) <6>
6 Mr. Game & Watch (31 top8, 16 top4, 8 wins, 103.5 pts) <5>
7 ROB (25 top8, 21 top4, 5 wins, 100.5 pts)
B Rank
8 Donkey Kong (11 top8, 13 top4, 7 wins, 79.5 pts) <10>
9 Wolf (16 top8, 16 top4, 3 wins, 71 pts) <8>
10 Pikachu (5 top8, 15 top4, 4 wins, 67.5 pts) <9>
11 Falco (13 top8, 11 top4, 4 wins, 59.5 pts)
C Rank
12 Pit (9 top8, 9 top4, 4 wins, 51.5 pts) <13>
13 Olimar (14 top8, 13 top4, 1 win, 51 pts) <17>
14 Ike (13 top8, 9 top4, 3 wins, 48.5 pts) <12>
15 Fox (8 top8, 11 top4, 2 wins, 47 pts) <16>
16 Lucario (5 top8, 8 top4, 4 wins, 46.5 pts) <18>
17 Kirby (6 top8, 10 top4, 2 wins, 43 pts) <14>
18 Toon Link (8 top8, 9 top4, 2 wins, 41 pts) <15>
19 Lucas (6 top8, 9 top4, 2 wins, 40 pts)
20 Peach (5 top8, 7 top4, 2 wins, 33.5 pts) <22>
21 Zelda (5 top 8, 5 top4, 3 wins, 32.5 pts) <27>
22 Ice Climbers (6 top8, 8 top4, 1 win, 32 pts) <21>
23 Sonic (5 top8, 4 top4, 3 wins, 29.5 pts) <20>
24 Zero Suit Samus (6 top8, 6 top4, 1 win, 26 pts) <23>
D Rank
25 Luigi (4 top8, 4 top4, 1 win, 19 pts) <24>
25 Sheik (4 top8, 4 top4, 1 win, 19 pts)
27 Diddy Kong (9 top8, 3 top4, 1 win, 18.5 pts) <26>
28 Mario (4 top8, 3 top4, 11 pts)

0>
29 Bowser (2 top8, 3 top4, 10 pts) <28>
30 Ness (4 top8, 1 top4, 1 win, 10 pts) <29>
31 Captain Falcon (3 top4, 9 pts)

2>
32 Ganondorf (5 top8, 2 top4, 8.5 pts)

3>
E Rank
33 Jigglypuff (3 top8, 2 top4, 7.5 pts)

1>
34 Pokémon Trainer (2 top8, 2 top4, 7 pts)
35 Link (2 top8, 1 top4, 4 pts)

4>
35 Samus (2 top8, 1 top4, 4 pts)

4>
37 Yoshi (1 top4, 3 pts)