I'd recommend
ZDoom. It's a free source port of the Doom Engine which adds all sorts of nifty graphical/gameplay bells and whistles - including the ability to play at high resolutions and completely customize controls (I have mine set up like a modern FPS, WASD movement, mouselook, 'n' all). It works for all Doom Engine games, including Heretic and Hexen, and best of all, there's no messy configuration or even installation required; you simply download and extract the program, plop the games' .WAD files into the same folder, and run the ZDoom.exe.
So, yeah. ZDoom.
