Deus Ex is not a shooter. Its a role-playing game with a camera set in the protagonists head, most of the time.
As for actual shooter games: Doom. Maybe Duke 3D.
I have not spent nearly as much time on any other game as I have Doom. Duke3D was superior in many ways, but I dont think I spent as much time playing it.
The good thing is both can be played in a Win32 environment. I have Doom 95 along with the WAD's for Doom 1 and Doom 2, and I also have the Win32 executable for Duke 3D.
Good times.
Actually, Serious Sam 1st and 2nd were assloads of fun too. Huge, beautiful landscapes, literally thousands of monsters per level, sometimes a hundred or more on screen at once, pretty slick secrets (some of which were easy to find and funny as hell) excellent variety of monsters and many cool ways to kill them. The guns went back to old-school where you didnt have to reload most of them.
I think I change my vote to Serious Sam.