Season one is unique - the finale is a single episode.
Seasons two and three have similar structures: A two-part finale. They have something else in common, too. I remember when I first watched I knew immediately that each was going to cop out on its own darkness. I know I've complained about this before. In Becoming, Buffy's boyfriend dies, her mother kicks her out, she's expelled from school, and she's wanted by the police. By the third episode of the next season, all that is undone, and there isn't even an explanation for why she is no longer wanted by the police. Watching Graduation Day, I knew the minute Buffy decided to go after Faith with a knife that this would have no real effect on Buffy's long-term arc. Sure enough, it didn't.
Season four is also unique. Like season one, it has a single finale episode, but unlike season one, it follows this up with an experimental episode.
Seasons five and six - not-so-coincidentally my favorite two seasons - also have a similar structure. A non-finale arc episode (Tough Love, Seeing Red) ends with an event (Glory finding out Dawn is the key, Tara's death) that kick-starts a three part finale. I found these three-part finales far more exciting than the two-parters of seasons two and three. The plots are driven by villains with clear and simple goals. Willow's decision to end the world always made more sense to me for her character than Angelus' decision to do likewise. Glory doesn't even care about the world, she just wants to go home. I felt Angel was better as a villain when he was focused on stalking Buffy - the end of the world just seemed like a distraction, whereas for Willow and Glory it was the whole point. And the conflict in seasons five and six is still personal for Buffy - Glory wants her sister, Willow is her best friend.
Season seven is another unique season: the multi-part finale could be said to start as early as Dirty Girls. This one is too complicated for its own good, in my opinion. gabrielleabelle tried to make sense of the timeline here, but it still confuses me.