The key ones I think are a must to participate in any sensible and adult crpg conversation (objectively since I greatly dislike some of these but they are necessary for any self-respecting crpg enthusiast's repertoire) would be - Dark Sun (either), Darklands, one of the Realms of Arkania Trilogy, Ultima 7, Albion, Fallout 1 or 2, Wizardry 7 and/or 8, JA 2, Planescape Torment, BG 1 or 2, Arcanum, ToEE, NWN 1 or 2, Lionheart, VtM: Bloodlines (action game with heavy rpg elements but still necessary), Gothic 1 or 2 (same as Bloodlines), any one of the Spiderweb Software games, one of the Drakensangs, Divine Divinity, Blackguards 1, D:OS, WL2, MMX, AoD, Underrail. I am also including Aarklash Legacy since there is absolutely no reason any enthusiast, especially people who defend or praise the horrible, awful IE games combat, to have not played the only RTwP game that has somehow magically actually made the only good RTwP combat system ever.
Highly suggested games - Daggerfall (the last PC game Bethesda made), one of the Goldbox games (preferably a Buck Rogers, granted they are not considered gold box but they used the engine, and they had a chargen and dev as well as combat ((that was has some very interesting checks and balances never again seen even though it was a very good combat/rpg system)) far superior to any of the D&D goldbox games). One of the Martian Ultima games. Mask of the Betrayer because people seem to piss their pants over it. Ultima Underworld and one of the Lands of Lore games so you can see exactly when rpgs started putting form over function, dumbing down gameplay, and pandering to the future console crowd. Thunderscape. EOB trilogy (2 or 3 preferably). Betrayal at Krondor (started the highly narrated, scripted, story heavy craze that is still all the rage today). Quest for Glory 1-5 (preferably the EGA version of 1 or 2 since they are far superior to the remake of 1 or 3-5). Dead State. I guess console trash like the Kotors, Dragon Age, Bethesda games after Daggerfall, the Witchers, whatever else created a lot of hubbub with the console crowd. I probably missed a couple that could be considered important to understand/appreciate the genre. I left out a ton of good games that I love that really didn't put anything else on the rpg table.