I've compiled the following list of all of the leakers known to me. Feel free to continue to update it or add it to the OP. I think it's a nifty little summary.
1) Neogaf E3 Leaker
Claims: Wii Fit Trainer, Little Mac, Animal Crossing Guy, Pac-Man, Mega Man, and Miis were to be shown at E3 2013.
Credibility: Obviously, this takes a hit in that we only saw three E3 characters. He also didn't refer to Villager by his canon name. Still... he predicted Wii Fit Trainer.
Other: Interestingly enough, he's only made that one leak. He isn't like the others in that he hasn't made any other claims.
2) InfiniteGTX
Claims:
Credibilty: Pre-Brawl, the same fellow leaked playable Krystal, decloned Ganondorf, and Poltergust Luigi.
Other: He deleted all of his pre-Brawl "leaks" as soon as they were discovered and claims that he has a new source now.
3) WiiLuvU
Claims:
Credibility: He had previously revealed that Wii Sports U was a pack-in title for the Wii U and that Pikmin 3 would be called Pikmin Universe.
Other: His threads have since been deleted. Whether or not he has admitted that it was false is currently unknown to me.
1) Neogaf E3 Leaker
Claims: Wii Fit Trainer, Little Mac, Animal Crossing Guy, Pac-Man, Mega Man, and Miis were to be shown at E3 2013.
Credibility: Obviously, this takes a hit in that we only saw three E3 characters. He also didn't refer to Villager by his canon name. Still... he predicted Wii Fit Trainer.
Other: Interestingly enough, he's only made that one leak. He isn't like the others in that he hasn't made any other claims.
2) InfiniteGTX
Claims:
1) Little Mac is in, was almost an E3 character.
2) Series themed Break the Targets.
3) Melee-esque Adventure Mode.
4) Mewtwo just got clearance and will function like Zelda/Sheik
5) Pokemon representative will be the next reveal.
6) Dixie Kong is playable.
7) K. Rool is a comical boss that fights you three times.
8) Chrom and Lucina function as a tag team and appear alongside Ike and Marth as the Fire Emblem Reps.
2) Series themed Break the Targets.
3) Melee-esque Adventure Mode.
4) Mewtwo just got clearance and will function like Zelda/Sheik
5) Pokemon representative will be the next reveal.
6) Dixie Kong is playable.
7) K. Rool is a comical boss that fights you three times.
8) Chrom and Lucina function as a tag team and appear alongside Ike and Marth as the Fire Emblem Reps.
Credibilty: Pre-Brawl, the same fellow leaked playable Krystal, decloned Ganondorf, and Poltergust Luigi.
Other: He deleted all of his pre-Brawl "leaks" as soon as they were discovered and claims that he has a new source now.
3) WiiLuvU
Claims:
1. Dixie is the only forbiddenseven character planned.
2. Yellow Devil is a stage hazard only.
3. Mac is in and was nearly an E3 reveal. King Hippo is an Assist Trophy.
4. Slight character customization is available. For example, Fox's blaster may change speeds and power.
5. Adventure Mode between Melee and Brawl.
6. Bosses are chosen with respect to playable characters---no Jr. because Bowser wouldn't fight him.
7. Mewtwo on the shortlist---near definite return.
8. No DLC of any sort.
9. King K. Rool is low-priority. He doesn't add much uniqueness is the reason.
10. Duck Hunt Dog is a playable character
11. No Ridley. No Jigglypuff. No Snake.
12. Paper Mario is in.
2. Yellow Devil is a stage hazard only.
3. Mac is in and was nearly an E3 reveal. King Hippo is an Assist Trophy.
4. Slight character customization is available. For example, Fox's blaster may change speeds and power.
5. Adventure Mode between Melee and Brawl.
6. Bosses are chosen with respect to playable characters---no Jr. because Bowser wouldn't fight him.
7. Mewtwo on the shortlist---near definite return.
8. No DLC of any sort.
9. King K. Rool is low-priority. He doesn't add much uniqueness is the reason.
10. Duck Hunt Dog is a playable character
11. No Ridley. No Jigglypuff. No Snake.
12. Paper Mario is in.
Credibility: He had previously revealed that Wii Sports U was a pack-in title for the Wii U and that Pikmin 3 would be called Pikmin Universe.
Other: His threads have since been deleted. Whether or not he has admitted that it was false is currently unknown to me.