I skipped Melee and Wii U/3DS. My favorite is easily Ultimate, and I don't think playing Melee and Wii U/3DS would change my mind on that one.
Which character will appear? Hard to say. Maybe something Monster Hunter-related, given how big Monster Hunter on Switch has been lately. Of Nintendo characters remaining, I'd say Akira Howard from Astral Chain. Or maybe the protagonist from Shin Megami Tensei V, since that's looking to be Nintendo's other big game this fall along with Metroid Dread.
Characters who should appear: Yuri Lowell, Scorpion, Sub-Zero, a Virtua Fighter character (as a full character, not just a Mii outfit), Adol Christin (Falcom is one of the few Japanese companies with zero representation), Amaterasu, Sir Arthur, Ryu Hayabusa (Koei Tecmo has surprisingly little representation given how much they work with Nintendo these days), Geralt of Rivia, Doomguy