Where it all began ....
For me, Doom started off the 3D shooter genre and has provided me with countless fond fragfests that I will remember forever.
Doom has been re-worked and an excellent source port can be found here: http://doom.wikia.com/wiki/Doomsday
This port gives you a more modern Doom and there are texture packs out there that you can add to Doomsday to make the whole thing hi-res. If you don't own Doom, buy it! Even in its original state, its an amazing game with millions of add-on levels and tools to alter the game. Simply the best game of all time and a recipe that many modern games producers fail to match.