Might be super obvious way of doing it, but I'd probably go with seasons 1 & 2 as first film. Seasons 3 & 4 as second. Then wrap things up with seasons 5 & 6. Most of season 7 would wind up on the cutting room floor where it belongs.
😉 However, I wasn't originally thinking of doing it this way. If you think about it, seasons 2 & 3 could really be movies all by themselves, but then you'd need more than 3 films, or third would be a white hot mess.
Makes sense to add bit of 1st season's set up to Angelus' storyline. While I get why season 4 is a transitional one, but if you think about it, high school and college can both be times of personal growth and exploration. Which is a good chunk of both season's personal story lines. Agree with
@Priceless losing Angel in third season. Angel should've stayed gone after season 2, or used sparingly.
As for final film, I'd say that most of the Glory story line would remain the same, but I'd either edit out Dawn, or change that narrative somehow. I'd have limited Trio, just long enough to bring back Vamp Willow, and de-chipped Spike as Big Bads. If I must include The First, I'd make them almost a smaller threat, but still bring in some of the potentials. I'd also not kill Tara, simply wound her. Just enough to get a mini reign of Dark Willow. I'd also overlap the 3 story lines. But still bit of in order of seasons. So not as to have them too jumbled together. And as an added bonus you get not just one, or two, but three Willows! Like shut up and take my money!
😀
Some of these (in thread)
could make for some interesting fanfiction, if done well. Just saying.
😳