That's one of the questions I couldn't answer.
5 / 6 / 3 / 7 / 4 / 2 / 1
Oh wow I just did.
It's pretty touch and go between the first four though.
I believe S5 was greatly written. It had great comedy, great drama, and also the best episode of the show ("The Body"), everything was going smoothly and was logical, prepared even : Buffy's death wish, Buffy/Dawn relationship... It also packed things up for S6 (Buffy becoming the adult of the family, Willow becoming an addict, Xander/Anya, Spike/Buffy, etc.). "The Gift" was written as a (great) series finale/100th episode and it shows by many little things (the Xander/Willow high school reference, etc.). It was like a movie, but knew when to stop and make loner episodes (when S7 didn't really know how).
S6 was great, too. Sure, it was flawed, we all know how : "hit you on the head" metaphores (magic = druuuugs), the ridiculous Trio, etc. But it had great character development, and showed us an all new face of the show, which I love.
S3 now... Well I didn't have it upper on the list because the high school years, not that much for me (even though I'm in a "nostalgic" time and watching S2 these days)... But almost everything about it was great : Faith, Bangel, the Oz/Willow/Xander/Cordy drama, Wesley, Giles, etc. etc. Of course it was greatly written. Still I think it lacked the maturity you could find in the seasons after.
S7, why does everyone hate it ?! I think it was a great season and a very good way to end the show. It was a new type of season, as I said earlier, it really focused on one story, much like a movie. Now okay my main complaints about this season is : too much audience-demanded Spike (it really wasted Spuffy for me), not enough character development for the Four Core, Killow (obviously... gyaaah), a not so badass vilain. But still, it was an empowering season which is good for a season focusing on power.
S4, well, I used to really dislike it, but now that I know it well and can stop focusing on the Initiative extravaganza (which was really badly written), I like it a lot better ! It had great character interaction, amazing loner episodes, a real atmosphere to it and lots of Tillow to love.
S2... Well, according to me, this season (and the show as I first watched it) really becomes good good goooood in mid-season (What's my Line, etc.). Before, it still smells of S1, with good and fun loner episodes but lacking maturity and getting not enough meaning (I mean surely it does have meaning but you get it pretty fast). And after, the arc is just amazing, "Becoming" is unbelievable and "BtVS" becomes "BtVS".
S1, I love "Prophecy Girl". The other episodes... Well it's as I said about first third of S2. Like it though, for the character development mainly, and the way Joss managed to put an arc there while still doing independent episodes.