Smash is basically a strategy game, with hundreds of tiny decisions that must be made every minute of gameplay (if you play smash at a high level) Competititve smash has an astounding metagame, and a very diverse range of players skill.
What I love about smash is that if you are more skilled than your opponent, you can use a weak character like pichu, but overpower a fastfaller like fox once you get inside his range and start attacking him if you can out mindgame the fox and you know your combos.
Yes at the highest level the top tiers and mid tiers are the only ones that will win, but 95 percent of smashers do not play at this level, so it doesn't matter.
Smash has 4 player capability, and a lot of options, and customization.. Item frequency, which items, which stages, different modes, and a fairly entertaining 1 player side to it aswell, break the targets, platforms, homerun contest, events...
Smash is just a unique game in every way, and thats why it is my favorite.