Firefly
Ren & Stimpy, every episode is a mindfuck
Cowboy Bebop
House
CSI (not Miami or New York, they are just fucking awful)
Star Trek: the original series
Star Trek: TNG, started out cheesy but got tons better
Stargate: Universe
So many great shows got shitty after a while.
X-Files, until Mulder started phoning it in around season 6 or 7
Scrubs when they switched to teaching, although technically it had a new name so I guess it was a different series.
Stargate SG-1, until O'Neill started phoning it in, and then left
MASH, got too preachy
Battlestar Galactica (remake) started out amazing, turned shitty near the end.
Simpsons, Family Guy, Futurama are all great, but they aren't consistently great. Some episodes make me laugh my ass off, some are just meh.
Sopranos was up and down. Some episodes were amazing, some were just meh.
Lost was interesting, but mostly a wtf.
I haven't watched Mad Men or Dexter, but I'm looking forward to seeing them from the great reviews.