I'm going to have go with the following:
Dungeon Master (video game) - Wikipedia
Tales of Aravorn: Seasons Of The Wolf on Steam (steampowered.com)
SKYHILL on Steam (steampowered.com)
Marble Age on Steam (steampowered.com) (haven't beaten the remastered version)
DROD RPG: Tendry's Tale on Steam (steampowered.com)
I play a lot of RPGs and strategy games. Sadly, I usually don't complete the really hard ones. I wish I could put Angband on here. I especially wish I could Bard's Tale 1 on here, but I used an item to open a door and then stupidly saved the game without the item.
Edit: Forgot about Sunrider: Mask of Arcadius on Steam (steampowered.com)