6. Season 4
Why It Was Great
It gave us Hush, which stands as one of the most inventive, memorable, terrifying, and just brilliant episodes the show ever produced.
The season finale is an all-timer too, a meaningful exploration of the characters' psyches and personas, while also foreshadowing so many major plot beats to come (from Dawn to Joyce's death and many more, it's quite crazy). It also establishes Spike as a main character who can't kill, and brings together Tara and Willow in a truly sweet, natural way, giving the show - and heck, TV - one of its best romantic relationships. It's Buffy moving towards adulthood, a necessary move even if there were a lot of growing pains.
Why It Wasn't
Overall, the season was a bit of a mess, albeit it's easy to see why. The great high school show was no longer in high school, while key players Angel and Cordelia had departed Sunnydale. It was a major transitional phase, and perhaps shouldn't be a surprise that it really stumbled.
Riley is the worst of Buffy's major love interests, with very little to make him a character of interest; the Initiative could've been cool, but wasn't well handled, and Adam is perhaps the worst, most underwritten and least interesting Big Bad the show ever produced. It also gave us Beer Bad, which is probably the worst ever episode of the series.
Best Episode: Hush (S4 E10)