DS9 S 4 and 5
TNG seasons 3 & 4
Supernatural Seasons 4 & 2
Lost S 1
nuBSG S 1
Walking Dead S 1
Downton Abby S 1
Sherlock S 1 & 2
X-Files S 2 & 3
Game of THrones S 1
TOS S2
Dexter S 1
Those are the ones that just stand out, there are many that almost make but don't quite for various reasons. THose are
VOY S 4 (easily the best and the first time the environment felt different then the alpha quadrant)
ENT S1 & S3 (season 3 is better but the Xindi was never an arc I wanted to see), but I really, really liked 2/3rds of season one a lot. I liked that for the crew everything was new, from the joy of walking on a planet, to seeing a comet, to making first contact for the first time. Trek has never, never managed that type of thrill before. OF course it also had flaws (plots that would be new to the crew, but not to the audience, and with VOY the cast I just as a whole didn't care that much about). I was much less pleased with S 4, as it had stories I wanted to see for example, but all shoved down my throat, and in manner that didn't feel organic (seriously the show should have done those episodes over at least two seasons not shoved into one), and didn't care for the change in colors at all, not one bit).
B5, great premise, some great acting, some terrible production work and some really poor acting and dialogue as well. If it wasn't for issues with dialogue and certain actors seasons 2 & 3 would be on the list above. Add season 4 for three seasons of almost great.
X-Files S 4 & 5 and lets just watch the show get worse and worse.
Buffy Seasons 2, 3, 5. Great characters, but hampered by being utterly unbelievable, and two many characters that were quick on the sound bite. Not everyone should be.
Angel S 2, 3 and 4 Darker then Buffy, thank you. larger town that you can forgive soem of the insanity happening. But still loses out on classic status.
Prison Break S 1 (Add a touch of Oz and it would have ben on the list above).
Heroes S 1. Poor pacing, but give it about 6 episodes and it really starts improving, and does so right until the last few minutes of the season ender. Then its all down hill from here.
TOS S 1 (so close to classic)
NuBSG S 2, 3 and 4 (almost, almost there).
DS9 S 2, 6. Almost almost as good as 4 and 5.
TNG S 5 & 6, both have some really awful stretches that took them out of great status.
Supernatural S 5 (let me edit out 3 episodes and it would be on the list above).
Lost S 4 and 5 (almost, but not quite)
Dexter S 2
Roswell S 1
THose seasons especially the first set I can watch almost anytime. All I consider strong to great seasons of TV.
Remember when Hiro lost his powers for no damn reason and was wandering around Las Vegas looking for a sword?
