here are my favorite iwads, and why I like them:
Doom - weapons chainsaw (a friend of mine used to own a mcCulloch eager beaver years ago, which is the doom chainsaw, it was a slightly newer model however) and the shotgun.
Doom 2 - ssg, additional monsters, bigger levels, slightly more fun.
TNT - the music and some of the level design.
Plutonia - nothing really, its more or less coopable though.
FreeDoom - music rules, its free and open source.