Here are my seasons in order: 1, 4, 2, 3, 5, 6, 7
Season 6 and 7 are really a toss-up, because I HATED the Lorelai and Rory split and the long-lost daughter story lines in season 6, but loved all the Luke and Lorelai couple moments we got in the first half. For season 7, I HATED the Christopher and Lorelai stuff, but loved that they did away with him as a romantic possibility forever at the end of it and I really enjoyed the last part of the season. So ... yeah, a toss up between those two for last place. If I could break up those two seasons, I'd put the last half of season 6 at the very bottom of my list (with the first part of season 7 right behind it). I will say, though, that watching season 6 is much more depressing for me than watching season 7.
As for my favs, season 1 is like a big, warm blanket to me. I love it so much. Funnily enough, I didn't even start watching the show on a regular basis until season 4, but my first introduction to the Gilmores was through those Gilmore Girls Beginnings reruns of season 1 on the WB and those reruns comforted me at a time when I needed it most. Overall I love pre-Yale Gilmore Girls more, but I love me some season 4. I love seeing Lorelai making her dream of the Dragonfly come true, I love that Rory didn't have a love interest for most of the season, I love the Luke and Lorelai stuff at the end SO MUCH, and I even liked Jason (as a pre-cursor to Luke only).