How about...Season 1 as the best, because the series started then, and Season 7 as the worst, because it ended then (at least as far as we knew at the time)
Alternately:
Season 6 as the best, because Buffy finally came to terms with her life, her duties, as the Slayer, and her family, and began to see that she actually could and, more importantly wanted to, find a balance in all of it. She didn't have to be JUST the Slayer, JUST the big sister/parental figure, or JUST the harried bread winner. She didn't have to let a single aspect of her life dominate all of the rest.
Season 4 as the worst, because Buffy was completely lost in a new environment, and having to face all of the real problems of life for the first time, and, frankly, in my opinion, most of those issues were either glossed over or were portrayed badly.