Trying to think about this... for a while my biggest 'shames' were never having played a Pokeymans (other than Snap), and only playing an hour or two of Kingdom Hearts, and none of Yakuza even though I own nearly the whole US-released series (still missing Kiwami 2). But in '18 I beat Pokemon Yellow and KH1, and I'm really far into Yakuza 7... but here are some other series I never played or have only played a very little bit of:
Monster Hunter - I've played maybe 10 minutes of the 3DS game at NY Comic Con one year, but nothing beyond that. I'm not so much a multiplayer/online gamer so the main appeal of the franchise wears short, though I do love the monster and environmental design.
Civilization - long running series but I don't particularly care for these types of games, just seems super dry and uninteresting to me... I think the SNES Sim City is like the only hardcore simulation game I ever got into.
Baldur's Gate - now I'll readily admit I'm not a big western RPG guy but I've played at least one game in most of the big guns - Mass Effect, Ultima, Witcher, Fallout... but never played any of the Baldur's Gate games (Planescape is the only Infinity Engine game I played). Just never got around to them, though if I'm craving a WRPG I'd want to play through Witcher 3 and Disco Elysium first.
Elder Scrolls - well, I've played a few hours of Oblivion (made it out of the intro dungeon and I think closed one of the gates), but never really bothered with this series. I'm not a fan of the excessive jank of Bethesda's games tbh.
Might and Magic - think I played a bit of a Genesis port of one of them back at Funcoland, but dungeon crawlers have never really been my jam, even today. Probably has some of the best aesthetics of the early wrpgs though.
Diablo - only played a bit of the first one at MAGFest a few years back. Would the D2 remake be a good place to start?
Phoenix Wright - kinda missed the boat on this franchise, truthfully I did not get into the VN genre till 999, I'm not big into law drama, dunno how much law drama+anime wackiness would draw me in.
Warcraft - I played quite a bit of Starcraft in the late 90s-early 00s with friends, but none of them were into Warcraft for whatever reason (high fantasy setting maybe?). Also don't play MMOs so I missed out on WOW.
Battlefield - not much for the super srs bzns military shooters, playing COD4 and Black Ops back in the 360 days was enough for me