Quake II is one of my favorite games of all time, and definitely the best shooter I've ever played.
I've completed it dozens of times, in many different ways - various difficulties, god mode, single weapons and unlimited ammo.



I know this game like the back of my own hand by now, yet it still gives me a lot of fun.
I recently found this awesome KMQuake2 graphics mod, that changes the game completely. It uses the Quake III engine, includes various visual effects, better textures, and native widescreen support with all modern resolutions.
I play the game at 1920x1080 with AAx8 and AFx16, and it looks awesome, considering I used the software renderer for a long time, because the standard OpenGL renderer in Quake II doesn't work under Windows Vista/7.

In this video you can see my favorite type of gameplay - walking through the entire game with a single gun, rarely letting go of the trigger. Using the Chaingun reminds me of the scene in Predator where they cut half the forest down looking for the invisible alien. The "mower" is my favorite weapon ever made, and I always get a little fizzing sensation whenever I hear this gun in a game or a movie.

The mod with instructions can be found here - http://www-personal.umich.edu/~jimw/q2/
There are other mods available as well. I tested them all, and they either looked worse, or didn't work at all.