I have a hard time ranking the seasons, mostly because my rankings can change depending on the day, and what I've watched recently. It is pretty fixed, though, that while seasons 1-4 may move around in my ranking, the bottom three spots always go to 5, 7, and 6, in that order.
If forced to choose I would say:
Season 2: I have the hardest time choosing between 2 and 4, but 2 has the edge because Rory is still in Stars Hollow, the Lorelei/Rory relationship is in it's golden years, and yes, the arrival of Jess makes for some classic and rewatchable episodes.
Season 4: I just love season 4..I find myself rewatching it more than any other season. I love to watch the construction and opening of The Dragonfly. I love the "trilogy" at the end. I also think it might actually be my favorite season for Rory. It's the only season where she is single for most of it, and I like that it has lots of "low drama" storylines ( like the study tree, and the ballet review) that focus on her as an individual.
Season 3: Has many of what I consider the most memorable GG episodes, but also has some things that annoy me. Gets the edge over 1 just because I tend to think of seasons 2-3 as the years when GG was in it's best, most natural, rhythm.
Season 1: Just ends up here because I like the overall "feel" of the other seasons a bit more, but I do love it, I think more than most.
Season 5: Still feels like GG, and I love the L/L stuff, but the end of the season (Kropogs!), is where things start to turn.
Season 7: The beggining is a little rough, but it's heads and tails above 6 for me. In fact, the last half of the season, starting with I'm a Kayak.., started to feel more like classic GG then anything the show had done in quite awhile.
Season 6: I don't mind the rift..too much. It's the second half of the season that's awful for me. I am a Lorelai girl, and her spark, for lack of a better word, is gone. The episodes are painful to watch and it just feels so mean spirited.