Classic Doom 1 and 2. There’s gotta be over 100 levels if you count TNT and Plutonia, which I think were sold as Final Doom? Anyway, if you just get the base games for 1 and 2, there are thousands upon thousands of community made maps, including some total conversions, so you can play new Doom content until it physically pains you to continue.
Of course, I feel obligated to mention that even though it would be super easy to pirate the WAD files and play with a free modern source port like GZDoom, like absolutely trivial to find copies of DOOM.WAD and DOOM2.WAD floating around the net, probably showing up easily on Archive.org, but… Um… Where was I going with this? Oh, right, don’t pirate. Cheap on GOG last I knew.
QuakeWorld and old school Doom for FPS, Beyond All Reason for RTS, Dungeon Crawl: Stone Soup for roguelikes, Hedgewars for a Worms-like.