Good old classic is Ghost in teh Shell movie + Ghost in teh Shell - Stand Alone Complex + Ghost in the Shell - 2nd Gig - There is in fact much more Ghost n the Shell stuff but these are my favs... once you get started you may not stop.
This is a very good show imo.
Also if you like classics Then Captian Harlock and Robotech still rule.. animation is a little old but the shows themselves are very good.
If you like Robotech you may want to move into the original jap version Macross... there is a load of Macross stuff all the way from the early 80s to now
Also you may want to check out GUNDAM.. again this gose from 70s all the way to now.. so there is tons of stuff to look through.
Try having a browse of
http://anidb.net (anime database) it has a great index of all things animae and i you register you get acess to ed2K links for all of it.. simply search by catagories.. though this will also list all films and OVAs and the like.
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Oh if you want a really good one that is a bit out of left field try out FLFC. It is by Yoshiyuki Sadamoto (of Neon Genesis Evangelion fame) It is a very very good show.. but like i said a little strange.
(ps - should prob move this thread into scifi fanatic cabal section)