Season 7 had the strongest start in my opinion, funny as it's one of my least favourite seasons. "Lessons" and "Beneath You", aren't too bad but "Same Time, Same Place" and "Help" are such strong episodes they definitely raise the average. Shame the season went down hill quickly after "Conversations with Dead People". Mid season 7 definitely bores me.
2. Season 4 "The Freshman" is a great season opener, Sunday's an awesome villain and it's great to see Buffy vulnerable in college, a relatable experience all round. I'm not generally bothered about how the opening episodes set up the themes of the season, but this one does well - the Scoobies are slowly drifting apart despite the fact they still help Buffy out. "Living Conditions" is one of my favourite episodes! So funny, I don't get how people dislike it! "Harsh Light of Day" and "Fear Itself" aren't too bad either, but the first two episodes for me bring the season off to a great start.
Weakest: A tie between Season 3 and 6 for me. The first four episodes of both seasons are ok, but not strong. They have great moments like Buffy being in LA, working as a waitress and being vulnerable in "Anne", the Scoobies' fight in "Dead Man's Party" and Faith's introduction in "Faith, Hope and Trick" ("Beauty and the Beasts" is a pretty weak episode IMO), but the rest of the episode isn't so strong. The season does pick up soon with solid episodes like "Homecoming", "Revelations", "The Wish" and "Gingerbread" and ends brilliantly.
Same with Season 6, besides scenes where Buffy comes back from the dead the episode "Bargaining" is a rather blah episode. Never liked even the idea of biker demons. "Afterlife" is similarly blah, besides the amazing final scene - but that doesn't make up for a blah episode. "Flooded" is meh. The season picks up from "OMWF" and ends brilliantly, but the start isn't spectacular.