I would be fine with multiple ways of unlocking.
Melee's method of having the player do various tasks is certainly the most interesting, as it encourages participating in the different elements of the game. I have to admit it was fun back in the day to mess around with different modes and options in hopes of finding something that may not have been known yet.
I do like having the fall back of playing X number of matches for those who only wish to play the multiplayer mode, even if it might take longer than other options, it doesn't force the player to have to use modes they do not want to. Plus, it was fun to just battle it out and then occasionally the winner gets the honor of freeing up the newest character.
A story mode unlock is fine, though a bit mundane in my opinion, I only unlocked one character in Brawl that way since I rarely touch the Single Player modes for anything other than getting the trophies. A story mode might be the easiest way to unlock characters without having to dedicate a long time before getting the one you want.
I've heard others make the case for having Amiibos unlock characters, which would probably be the fastest way to get specific characters and reward those who had already purchased the figures. Of course, some may see it as an unfair paywall, though as long as there's other, free ways of easily unlocking characters, I don't see the harm in it.