Daggerfall is a fun experience for the hardcore RPG enjoyer. It was released for free exactly as it was 30 years ago by Bethesda
on their website, and can be downloaded in a number of different places. It has a small community of enthusiasts in the modern day who mostly crowd around a
remaster of the game in the Unity engine, developing and playing mods for said remaster.
Fallouts 1 and 2 are well also well liked among RPG enthusiasts, with the turn-based gameplay of both being mostly the same, but the second obviously having a few small improvements. The first has a smaller world, but with a more consistant and darker tone, while the second is larger and a bit more goofy at times, more similar to later Fallout games.
I should mention that if you enjoy the OG Fallouts, there are also a handful of fan games that were made in their style. These games include
Fallout: Yesterday, an attempt to recreate the original cancelled Fallout 3 in an upgraded Fallout 2 engine, Olympus 2207, a Russian developed total conversion mod that is kinda difficult to find nowadays (the developer's site shut down a couple years ago for unclear reasons) but is fun if you can find it, and
FOnline, a small family of classic Fallout MMORPGS with a small but dedicated playerbase.
As an aside, although I personally don't enjoy its gameplay, shoutout to Morrowind which has its own small dedicated MMORPG fanbase via
TES3MP.
For other Genres, I am personally well acquainted with old-school FPS games, so I'll discuss those. There are the obvious id classics like the OG Doom games (
and their zillions of mods) and Quake (
Arcane Dimensions is a great mod btw), but a few sometimes overlooked classic gems would be the Serious Sam games. Serious Sam was an evolution in the boomer shooter formula where the more advanced hardware of the time allowed devs to make their players face off against huge hordes of enemies. Enemy counts in classic SS levels scarcely drop below 100 and are regularly many times more than that, yet with a large arsenal of powerful weapons, you never really feel outmatched.