It's Mega Man for me. Growing up there were three huge pillars of gaming as far as I was concerned: Nintendo Games, Final Fantasy, and Mega Man.
When Brawl was announced with Snake, introducing the idea that 3rd party characters could make it in, I thought Mega Man was a sure thing. And then he wasn't. The tears were bitter.
But then, Smash 4. A new chance. On the surface, I had hoped. But deep down, I wasn't sure it would happen. The trailer finished, went to black. If it's happening, it's here, I thought. And then it did.
The implementation of Mega Man is perfect in Smash 4. It's exactly what I wanted, faithful down to the animations. I couldn't have dreamed up a better rep. (Could've used a Protoman skin though, 'cmon.)
When the 3DS demo came out, I had a chance to finally sit down for an extended period of time with the character. I had already decided to main him no matter what, but I just couldn't get him to work that well against the level 9 CPUs. I kept playing, to adjust to the 3DS controls, but I started to have my doubts about his competitive viability. I couldn't approach, I could consistently kill. It was tough.
Then the 3DS game came out, and I don't know what was different, but Mega Man felt good. I just kept playing with him to get where I'm at now.
No regrets.
P.S. His down-air is the best down-air.