it's always a bit awkward when a question like this is asked with traditional 2D fighters right next to Smash Bros, as one could consider the games quite different
traditionally speaking I would probably pick Killer Instinct, simply because it has a soft spot in my heart. buttttt obviously Smash Bros. as a series has went above and beyond anything else to make extremely intricate fighting circumstances with a wide variety of moves unique to each character, items, varying platforming levels, etc.
Super Smash Bros. Melee is the gold standard for me, no fight in that game is ever the same and when you play it with more than 2 people it is somewhat addicting. I think what the Smash series also has done SO much better than any other fighting series, is provide stats and unique ways of play. If you play with a group you can play for a few hours and it will keep track of the number of KO's you have, victories, etc. all kinds of levels, tournament editions, a huge variety of weapons that you can specifically turn on/off (what other game has that! amazing feature).
I'm rambling but beyond specficially the fighting category, I would argue that a game like Melee is one of the best of ANY category because it offers so much
