I had tremendous fun with Might & Magic (not
the "heroes of" then-sub-series, I found those mediocre fun-wise, though the music was top-shelf stuff). 3 was my first and I had fun never actually finished it, but I exhausted 4 and 5 (they merge together as if 5 was a massive expansion pack; starting on 4's content is probably best). 6, 7, and 8 use a different engine but it's also a ton of fun and I found those were actually the golden age of the series for me (particularly 7). 9 was another new engine and an incomplete-on-release travesty and all the more tragedy because they never tried to continue that series again.
Baldur's Gate... it is the definitive CRPG experience, even today. I found the second one more fun, but part of the fun is having followed along from the beginning and grown into the characters, etc. Be prepared for a long
haul, but it sounds like that doesn't daunt you.
Planescape is also incredible though a lot darker (in my opinion, having recently been through the story of Planescape; not excessively dark, but dark).
Fallout 1+2 are also classic CRPGs, I definitely recommend playing the first then the second; not as much carryover as Baldur's Gate (by far) but it feels better that way.
If you're really into the CRPG stuff, "Arcanum: Of Steamworks and Magick Obscura" is definitely worth it. Not up there with Baldur's Gate for me but solidly enjoyable.
Depending on how much you like the strategy genre: Master of Magic is still my favorite strategy game. Master of Orion is also great but the Space 4X and Non-Fantasy Land 4X genres have fared far
better than the Fantasy Land 4X genre over the decades, so I don't feel it's as... necessary, in a classics collection.
Sid Meier's Alpha Centauri is my favorite non-fantasy land 4X game ever, even without the excellent expansion (which sadly GoG does not offer). It can bog down a bit late-game, but the terraforming options are incredible and the voiceovers on tech discoveries are a trip in multiple senses of the word.
Descent Freespace 2 was the most fun I've ever had in a space-flight-sim (though Tie Fighter was a close second, goodness how many joysticks I wore out playing Tie Fighter). Indeed, that genre seemed to just "stop" after that game; there've been other ones, but none I paid much attention to.
Jagged Alliance 2 is awesome.
Dungeon Keeper (I also liked the second one) is unmatched in its particular style of fun.
Oddly enough, I put Deus Ex down after about 10 minutes
I could go on but really shouldn't