Havent seen it. Heard good things about it though. Look up Gasaraki, Vexile and AD Police. Not they typical cutesy anime a lot of people think of. Also Elfen Leid. Actual substance there.
Cowboy Bebop was released on DVD this week, and is now on Funimation’s website and YouTube channel. It’s a bit dated (1990s) but is still considered by many to be in the top 5. (It’s noir-ish).
If you like romantic slice-of-life, CLANNAD and CLANNAD After Story are the best. Also online.
If you like SF and mindscrew, there’s Haruhi Suzumiya.
These are all adult seinen, PG or PG-13.