In each and every fighting game, I must not only enjoy my main as a character of their own franchise, but they must be reasonably fun. It does not hurt if said character is viable in the competitive meta-game either.
In Smash 64, I alternated between Link, Luigi, Fox, and Jigglypuff as those were my favorite characters at the time. The only Nintendo franchises I had ever heard of were Legend of Zelda, Mario, Star Fox, and Pokemon. Star Fox 64, Legend of Zelda Ocarina of Time, and Star Fox 64 were pretty much the only games I played outside of Smash. I loved Pokemon Red and Pokemon Gold, my versions of the titles just as much. Later, my mother's boyfriend bought us Stadium 2 as a surprise gift. Their play-styles worked for me as well. I never did play Smash 64 at a high level, however. So these were decisions based on franchise loyalty over all else.
In Melee, I maintained those four characters and added Ganondorf, Zelda, and Mewtwo in addition to the clone army (every clone in the game). They might not have been "mains" per se. At this moment, I was much better at Smash and I played more seriously with a play to win and have fun attitude. I was proud to main the Triforce. I just used Jigglypuff for fun when I felt like playing more defensively and Mewtwo when I was solely intending to try something different. These choices were based on fun above all else.
In terms of the clone army; slowly, but surely, I began to stop using Fox or Luigi as much. Instead, I dabbled into Falco and Dr. Mario respectively. I outright discontinued Roy as my sister began to main him and Pichu was more of a joke character (albeit, I maintain that he is better than Pikachu).
Brawl changed everything. Most of my mains were already low-tier, but Brawl ensured that each and everyone of them was either removed or bottom with the exception of "Young Link." I hated Brawl Link and decided to drop Link for Toon Link. It was hard after using Link for most of my matches. I still used Ganondorf and Zelda for novelty purposes. Jigglypuff was not so bad, I adamantly believe that her tier placement is incorrect. That being said, after a couple of years I gave up on her until the announcement of Smash 4 where I went back to Brawl and picked her up in anticipation for Smash 4. Lucario was my main at the beginning of the game's life-cycle (first 2 or 3 years) when I was learning. He was a great character to learn the game, so that is why I chose him.
At this stage in Smash, I started became more aware of competitive Smash and wanted a viable character. I started to like Marth's play-style from Melee and Brawl, so I decided to pick him up and Falco was much more manageable for someone with little tech-skill such as myself. These mains were based on viability above all else.
In Smash 4, I made note of my past decisions and mind-set to establish that I would consider a combination of attributes before deciding a main. I also declared to myself that I would not main characters out of habit unless I thought that they were worthy. I let a lot of nostalgia dictate my mains in (early) Brawl and viability in (late) Brawl and overall it was not enjoyable because of that. So everything had to be taken into consideration.
I played through the entire cast three times. Ganondorf, my previous all time favorite is not fun in Smash 4. So I decided to drop him. I will still use him for fun, of course. But I cannot main him any longer. Robin and Lucina were great characters in Awakening and I love them, but they are not my style. Marth is superior (and more fun) than Lucina in almost every way, and Robin is unfortunately not my style. Marth was my main in Brawl, but I am not fond of his character, so I am going to pass on him too. Dr. Mario has my favor, but his horrible recovery attenuates my enjoyment of playing him... so I cannot main him either.
That leaves the characters that I am interested in: Falco, Jigglypuff, Little Mac, Ness, and Yoshi. Ness was my main in Project M as he was such a step up from previous version and I actually had a lot of fun. So I am going to continue using him. Yoshi was always one of my least favorite characters, but he has an unprecedented amount of "flow." Little Mac is a "for fun" type of character given the nature of his move-set. But nevertheless, he is my favorite newcomer. Falco and Jigglypuff have history with me, they are great fun, and they have decent tools.
Those are the characters I am most interested in; however, anything can change. I decided to keep Falco as an assured main to work at as a lot of people underestimate him. Jigglypuff, again, for a more defensive play-style. Ness and Yoshi are top ten viable and fun. It is hard to beat that.
Tl;dr: It depends. Personally, I prefer the air.