Maybe the first season and to a lesser extent the second, but taken on average the third and fourth seasons really drag it down IMO.
I like to think of a "great" series as one that has the whole package... acting, directing, writing, cinematography, music, etc., and maintains at a high level for most of its run. Of the ones I've found the time to remain loyal to:
LOST - Probably my favorite of all time
Fringe - My new favorite (stop trying to kill it Fox, you bastards!)
X-Files - My favorite previous to LOST
Dexter - Did I say Fringe was my new favorite?
Sopranos - Fuggetaboutit
My So-Called Life - Way underrated and dead before its time
There are plenty of other good ones on right now, but they dont quite belong in the list above. Im sure Im also forgetting a few from the past