To The Moon is IMO the best RPGM game out there, but you've already played it. Quintessence: The Blighted Venom is a freeware game by them that's definitely worth checking out. http://www.freebirdgames.com
My #2 favorite is still Eternal Eden by Blossomsoft. Sure it may not have the production values of more recent games and the writing is a bit stilted, but it still offers one of the best gameplay experiences out there. I almost never do sidequests and Eternal Eden had me wanting to do all of them. The mechanic of "visible encounters that don't respawn" was brilliant and made backtracking and exploration an absolute dream. The puzzle dungeons were great too. I still fire that baby up every once in a while. http://www.blossomsoft.com
Best production values definitely go to Rose Portal Games' Whisper of a Rose and Sweet Lily Dreams. I haven't played much of Sweet Lily Dreams, but I did enjoy Whisper of a Rose. Don't be fooled, though, because Whisper of a Rose may look sweet but it's thorny and will kick your butt. http://www.roseportalgames.com
Aldorlea's games can be hit or miss (i.e. Laxius is a VERY acquired taste, Sylia just isn't that great) but Millennium is easily my favorite series by them. The visual style, the music, the story, the characters, the charm... all of that tickles my mojo. That's worth checking out.
Aaron Walz is one of my favorite new composers on the scene and his music is why I like the Aveyond games a lot. The first couple of Aveyond games haven't aged super well, but 3 is still solid.
Deadly Sin: Shining Faith by Dancing Dragon Games is also quite good. Refined visual style, good music, and I enjoyed the storyline. They have a steampunk game called Skyborn coming out soon that piqued my interest (and considering how ubiquitous RPGM games are and how some have just been terrible lately, this is no easy feat). http://www.dancingdragongames.com
EDIT: And though it's not an RPG or an RPGM game, it's worth checking out. It's called EmberWind and it's a Ghouls 'n Ghosts style action side-scroller. It was made using its own proprietary engine and it loads of fun. http://www.timetrap.se