Some I remember enjoying, some already mentioned:
Escaflowne (not the butchered Fox Kids version)
Haibane Renmei (mindfuck but so beautiful)
Serial Experiments Lain
Although I watched all of these when I was a closet otaku. I was obsessed with Japanese video games, shmups primarily (if anyone can get a hold of Geppy-X it's fucking hilarious and is self referential to mech anime and games it's so good) and bought and sold rare games across a few different game...