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.
The problem is they lose their email address, which is tied to just about every digital account they have. Losing your email can royally fuck you. Many sites send an email to the old email of you try to change it, so you’ll have to get in touch with support to get it changed, and then support will want proof it’s your account somehow, and the whole process is gonna take days or weeks to fix all your accounts. God help you if you forgot any passwords because now you can’t login or even reset the password since that goes through email.