Another S7 vote here. I think it was the most consistently excellent season (with one major slip up in The Emperor's New Cloak, I think) with some of the most amazing material I'd ever seen on DS9. It also managed to end on a very high note with the final chapter although Extreme Measures was somewhat disappointing and the final confrontation between Sisko and Dukat wasn't what it could or should have been. Still, I think the final chapter marked some of the most amazing material DS9 had ever done IMHO.
This is how I'd probably rank the seasons:
1. Season 7: One of the most consistently excellent seasons IMHO.
2. Season 5: Extremely consistent again with some great material but not quite on the level of S7, I think. In the end, this one almost deserves to be second on the basis of having produced Let He Who Is Without Sin alone...
3. Season 6: Has some of THE most amazing episodes this series has produced and a great start to the season but many stumbles along the way. This really comes off as a bit of a mixed bag, I think, and not as focused as it should have been after S5 and the setup at the end of that season.
4. Season 4: Very consistent season and where DS9 IMHO just skyrocketed compared to TNG and the other Treks. This really blew me away back in the day and still contains one of my all-time favorites (The Visitor). However, it's not as focused as later seasons would be which IMHO puts it below the final three though it's still a great season. Oh, and in my book, this one has no real clunkers although The Muse isn't all that interesting.
5. Season 2: For me, clearly the best of the early seasons. This one was pretty focused for Trek or DS9 at the time with some great material dealing with Bajor that I really, really liked. However, and that's my major complaint about all of the early seasons, it never quite reached that level of 'special' IMHO that later seasons and episodes would achieve. I think one of the reasons is the overall look and feel, as superficial as that might seem, as well as the music. It's just a bit too flat and doesn't support the great material that's here enough.
6. Season 3: This has some tremendous material in Improbable Cause and The Die Is Cast as well as some other great moments but, boy, is it a bumpy ride overall. For one, a lot seems to be shifted around (especially compared to what was done in The Search), secondly they really had quite a few bummers and/or pretty lame episodes in the mix, unfortunately. This season would have needed to be a lot more focused. I think a great thing would have been mapping the first half of the season to deal with mostly Bajoran matters since they were very strong in S2 but gradually shift to the Dominion. I know that after the events of The Jem'Hadar people were wanting to know what was up with the Dominion but I think this is something that could have been revealed far more effectively and slowly throughout the season. Of course, none of that would have worked in practice since they were keen on introducing the Defiant
7. Season 1: Definitely the weakest season IMHO. Many episodes come right of the heels of TNG, and this is always when DS9 was at its weakest IMHO (ok, that and most of the Ferengi stuff and when Behr and Beimler were smoking whatever it was they were smoking at times - still love you guys!

). However, this season clearly shows the promise of what's to come in the opening as well as the final episodes the latter of which did a great job laying the groundwork for S2 (a shame not all of S1 was spent doing that). Emissary, in my book, is actually one of the best DS9 episodes of the entire run of the series. I think that's an amazing feat considering it's the pilot. This is the only episode of the first three seasons (with the exception of Improbable Cause and The Die Is Cast, probably) that feels 'special' in the way I mentioned above. It really embraces the powerful premise and gets to the heart and sould of these characters, particularly Sisko. It's very fitting then that the weakest material surrounds the technobabble relocation of the station but that just doesn't bring it down IMHO. Ok, sorry for the Emissary detour

. It just fascinates me how much I like that episode and how it has grown on me over time. Part of that is probably because it wasn't until a short time ago that I really understood what the Prophets were saying to Sisko when they said, "You exist here".