Well, there are a few ways to judge this. It's the best roster just by virtue of having the most characters, sure, but I can break it down too:
What the roster has that I wanted: A+
Jigglypuff is still in and my third Melee main is back in Dr. Mario. I play a ridiculous ton of characters, but honestly, even if it was just Jigglypuff, Kirby, Falco, and Dr. Mario, I'd have to give a perfect grade.
What the roster has that I didn't want: D-
Until now I haven't hated any Smash character, but this roster is loaded with characters who I want to punch in the face. Sometimes that's in a good way (Duck Hunt Dog), but mostly in the bad way like with Bowser Jr., the Miis, and four others.
What the roster doesn't have that I wanted: D
I can completely ignore that Dixie Kong, Daisy, Waluigi, Isaac, K. Rool, etc. would have been reasonable picks and didn't get in, but I still have to hand out a D because Ice Climbers, Lucas, and even Ivysaur were some of my favorite characters to play, they were already made, and they're gone.
Ice Climbers especially hurt since they've been there for two games.
Newcomers only: C+
I only like five of the newcomers, so a C+ might seem a bit high, but this is quality over quantity. Mega Man's such a huge name, Sakurai surprisingly made the correct call with Robin and Lucina instead of Chrom, Palutena's moveset variety is intriguing, and I had negative interest in Pac-Man until his crazy moveset sold me. All of these characters except Lucina really stand out in terms of feeling unique too.