I have to say I divide it into two groups, High School and No High School. And I prefer No High School, because all of my favorite episodes except for one (Witch) are there, and also because character develoupment seems to be deeper... we see that Buffy isn't always a blonde with a calling, and Willow isn't a total nerd, and Xander isn't dumb all the time, and Giles isn't a funsucker (HELLO SOMBRERO!). The effects are better as the show goes on, the romance changes into something more intense... oh, and there's always the
beautiful point of No Angel.
In your divisions, I would say the college years are likely my favorite... as much as I love Season 6 I hate Season 7... but though I adore Season 4, I have no gripe with 5. So College years win.
Yay! I feel so unique!