I was never a 'trekkie', not even remotely, ok, maybe remotely, as I did keep a bunch of episodes from the original series on tape for years and years.
But, I've not seen any of the series in full, so i can't judge what was best to my tastes.
I have seen maybe 20 eps of TOS... I watched a lot of TNG when it first came on the telly, but not much since... watched random eps of Voyager here and there as my nephew was into it when he was wee... saw all of s3 and 4 of enterprise...and i only caught ds9 a couple of times...So I guess my answer is TOS, because when i sit down to watch an absolute classic ep of TOS, with it's particular characters/actors, the music, the sound effects, it's overall atmosphere in other words, well that is my best time watching Star Trek, and by classic eps i mean... The Corbomite Manuever, A Piece of the action, this Side of Paradise, that one where the they go back into the past and pick up that airforce pilot, Amok Time, the Nazi planet one,...damn I need to get these on dvd at some point, cause i missed some classics, and i need to replace my tapes cause only a couple of them are still playable.