DooM 2 for me. Much as I loved The Ultimate DooM, the sequel offered so many more enemies (plus, the versatile DB shotgun), that I've replayed it far more times than TUD.
I'll walk out on the heresy plank and state that I haven't played DooM 3, DooM 2016, DooM Eternal, or any of the games that succeeded DooM 2.