Are they all dark? I could use something lighter after NGE.
They're all have one sort of dystopia or another for their setting, so yes, all dark.
Ergo Proxy is probably the only one not depressing of the bunch.
I can't really think of anything that would qualify as brainy and not dark at the same time.
Haibane Renmei maybe? There's still a lot of shadows lingering about, but overall it's pretty bright and hopeful.
Ah, right, Serial Experiments Lain. Not exactly brimming with joy, but not bathing in angst and despair either.
Ok, the more I think about it, the more anime I can remember which actually fit there. There's always some dark, mature themes in these, but it's only fair - it's hard to make a psychological comedy sitcom:
Mushi-shi(or Mushishi) - that's probably your best choice. Very calming, serene and thought provoking at the same time.
Kino's Journey(aka Kinoo no tabi) - shit happens in this one, but the overall feeling somehow stays uplifting
The Sky Crawlers - on the level of GITS movies most likely. I wonder if that's because it was made by the same guy.
Welcome to the NHK - now that's more of a >psychiatric< cases study rather than just psychological, but it's great nonetheless, and keeps a healthy dose of humour, somewhat in spite of the depressing lives of the protagonists.