I haven't seen all of Season 6 yet, except for select episodes that ran on Comedy Central. Same for Season 7. I have all the seasons on DVD, and need to get around to watching 6 and 7.
As far as individual seasons go, I'll go with 5, 2, 3, 4, 1. Season 1 wasn't very good because Danny Devito didn't join the cast until the following season.
When it comes to individual episodes, I love the "Gang Goes To The Jersey Shore" episode from season 7, "Lethal Weapon 5" from Season 6 and "The D.E.N.N.I.S. System" from Season 5.
Captain America's been torn apart, now he's a court jester with a broken heart/he said "Turn me around and take me back to the start/I must be losing my mind, are you blind? I've seen it all a million times"