-DC's best ongoing series? Green Lantern Corp was a good start. (Haha only one issue.) I can't stand the JLA. So I might say Detective Comics. However, there is like at least 15 books (No joke) focused on Batman being released a month, so it's hard to choose.
-the best non-Marvel or DC series? I'm partial to Flight from Image. Well, formerely Image. 100 Bullets is good though.
-DC's best limited series? Kingdom Come
-Marvel's best limited series? GLA: Misassembled and Phoenix Endsong are my two current favorites.
-Mainstream Marvel's ongoing best series? I'm partial to Cable and Deadpool. Awesome art, great stories that don't take themselves too seriously. Funny too. Daredevil has been really good though.
-Ultimate Marvel's best series? The Ultimates is where it is at. Awesome comic. Vol. 1 is exactly how any Avengers movie should ever play out, Period.
-Marvel's best ongoing series? Supreme Power (MAX Punisher is pretty good too.)
-The best series currently going on in all of comics? The Ultimates or Supreme Power. Depends on you flavore. Definately pick up Cable and Deadpool though!