With the way the modding scene is progressing, Ultimate would eventually top Smash 4 for me.
The filesize limit for most Ultimate files recently got lifted iirc, so you could have characters that go over the poly limit. You can already put music bigger than the original ones you're replacing, and that's a major plus for me.
All Ultimate really needs now is one-slot voice mods and that'll be it. Well, if I have a Switch that is.