I'm not saying this should be your main, but I believe that the best character to start with is Mario
. This is because it is my opinion that Mario transitions well into almost every other character and can naturally utilize the entire trinity of playstyles: attrition, combo, and gimp.
But before I give some reasoning, I think that a lot of choosing a main depends on what your goal is. If you don't care about this game and just want easy wins, Kirby has by far the best ratio of strength to practice time required. For the sake of this post, I'll assume that your goal is to be a well-rounded, solid smash player a year or two down the road. And for this I think Mario as a first character is the best choice.
There are three main playstyles: attrition, combo, and gimp. Attrition is how most of the smash series is played after 64. You rack up damage by using your characters best moves and options and then knock your opponent far enough offstage that they can't recover, or else find an opening for a small combo and use your finisher (dair > upsmash on Mario for example). Mario can play the attrition style very naturally with his solid hitboxes, decent projectile, and strong recovery. Since you can't really be gimped, racking up damage slowly is a safe style. The attrition playstyle relies on outspacing your opponent consistently. This may either be because you are a stronger player (overall or in the matchup) or else because your character's attacks have better hitboxes.
The combo playstyle is the hallmark of 64: stringing together moves close enough together that your opponent can only DI. Mario's combos are pretty solid, often using SH dair > uair into more uairs but he can also sometimes get fair or nair chains more horizontally. His upsmash finisher is exceptionally strong but even a bair or fair into edgeguard will often be enough.
The gimp playstyle relies on killing your opponent early by taking advantage of their poor recovery. Obviously, this style can only be used against certain characters but learning how to consistently gimp is essential for many matchups at a higher level. Mario gimps well since his throws are both relatively horizontal and his dair pushes downwards enough to make it effective even against strong DI. When Mario plays someone like Falcon or Fox, he will attempt a gimp whenever possible.
Since Mario can play the three main playstyles very effectively, getting good with Mario will naturally make you good at the most important aspects of the game. This allows you to transition and learn other characters quite easily:
- Luigi
: very similar moveset, combo forms are the same except for upB finisher, recovery tools are nearly identical
- Falcon
: Mario's uair combos provide a solid foundation to learn Falcon combos and much of the intuition overlaps
- Fox
: often considered a faster version of Mario, Fox can also play all the playstyles naturally though his poor recovery will be a challenge initially
- Pikachu
: Gimp style is remarkably similar (throw > offstage edgeguard or fsmash/dsmash). Additionally, strong aerial hitboxes, nair, jab grab, strong upsmash, aerial speed (and others I haven't thought of) are all shared traits
So if you believe what I've written, learning Mario will give you a great start on another 4 characters. You get almost half the cast essentially for free! And of course these are just the character who I think have strong similarities with Mario.
You might think that you could make this argument about any character but I really don't believe that to be true. Learning Ness doesn't give you a lot of insight into the other characters. He shares the DJC technique with Yoshi but they use it very differently and his main playstyle is combo/tech-chase which is rather unique. The most similar thing to Firo's Ness tech chasing with dairs on platforms is Josuke's Samus, but Ness and Samus are very different characters. Learning one doesn't really guarantee you'll be good with the other.
A few final things to recommend about Mario:
1. He's on the upper side of the middle of the tier list. This means that you can pick up stronger characters later and fully appreciate their strengths. I know a lot of people who start with PIkachu then don't pick up a secondary for years and years because everyone feels so weak in comparison
2. He's a less-played character which means you won't have to worry about playing people who have years of matchup knowledge on you 99% of the time (trying to learn Falcon by playing online Pikachu's was the worst)
3. He's slow enough that you won't stumble all over yourself while learning tech
Sorry for this long post, I've been thinking about this for a long time and haven't put it into writing before
