Another one I remember watching that nobody mentioned was Captain Planet. It's kind of fun to rewatch the cringy episodes about difficult issues like AIDS and Northern Ireland. My favorite was the anti-drug episode.
Alot of the shows I'm seeing in this thread are shows I remember watching, but not on Saturday morning. I was a big fan of Ducktales (of the cartoons I haven't seen since I was a kid, that's the one I'd be most interested in rewatching), but I remember that being on Disney Afternoon after school, and I remember G.I. Joe as a weekday morning show that I watched in the summer.
Maybe Nicktoons started out on Saturday morning, I don't remember. But I remember Ren and Stimpy being moved early on into the original Snick lineup, and that's what I associate it with.
Of the ones you listed, I remember Garfield and Friends being on right before Ninja Turtles.
I played with Mask toys but I don't remember the show at all, although I just randomly saw a clip of it in the last couple days and I can't remember where.