Pondering the Star Trek always starts off bad theory. I was looking over season 1 of TNG the other day when Andy started the survivor, and thought it was an even mixture of passable episodes and crap until it ended very strong. Four of the last seven episodes are very good. At quick glance, season two seems to be about the same quality wise, but the good episodes are distributed pretty evenly.
I actually don't hate the first two seasons of DS9 at all. Looking them over, though, they're certainly not strong. Season 1 has very little to offer. Season 2 has some good episodes, and it's where they start laying some groundwork for the rest of the series, but overall it's still average at best. After that, the show really turns bad ass.
I agree with most about Voyager. Some of the crew were interesting, and a helluva lot of them were awful. The Maquis thing was completely wasted. Just as bad in my opinion was the terrible bad guys in the first season. Those Kazon people really sucked pretty hard as villains. The Vidians were an interesting concept, but were mostly just creepy. The chick that defected was a problem as well. She was actually one to add some much needed tension, and they got rid of her. Furthermore, she was mostly annoying from the get-go. Betrayal by somebody that's already a twat has no impact, as opposed to getting screwed by a valued, liked and trusted companion. Add to that, Kes was both annoying and useless. Considering the effort they put into developing her and the fact that she was well liked among the crew, she should have been the one to sell them out. Regardless, once they replaced her with Catsuit-Girl, the show improved quality-wise.
Thus far, I'd say that they all started on a pretty low note and improved after the first season. TNG actually got rolling in the first season. Enterprise might well be the exception. I thought the first 2 seasons were generally pretty interesting, and it also felt like Star Trek. You got to see plenty of the the onset of what we got used to, weapons, transporter, tractor beams, etc. You were introduced to one or two of the older races. Some of the episodes were better than others, but rarely did they absolutely suck. Not bad, just rarely great. Season 3 was certainly better entertainment, and as pointed out the Xindi were an excellent concept, it just never actually felt like Star Trek to me. It was like any other sci-fi show on UPN, except not quite as good. From then on out they were just throwing out gimmicks trying to not be canceled. (And Worf's stance during Trials and Tribulations was fucking fantastic. I really hated that they had to go and screw that up with some silly story about genetic re-sequencing, or some garbage.) Personally, I'd say that it started off alright and tanked pretty hard towards the end.