Here's some of my favorite wads that left such a strong impression on me you can see their influence in my stuff to some degree or another depending on map:
Sunder - Every map has the progression, ambience, scale and music of a final showdown, something that inspires me greatly.
Going to Hell - WIP megawad that never got finished that i'd describe as "BTSX E1 without the cookie cutter level design".
Paradise - Has combat, aesthetics and music that inspire me like nothing else. To me it almost felt like playing a whole new game.
Lost Civilization - Gigantic explorational lush levels that make you feel like you're in a real place, without sacrificing the fun of gameplay.
Hurt - This is the only one of these that is one single map, and despite that it feels like something special. Even from looking at the screenshots you can tell that.
There's many others i'd recommend like TNT Revilution and Eviternity, but i think those above are on a whole other level. They felt like whole unique experiences for me.