Great question. I think Buffy is really solid overall, but there would definitely be a few things I'd alter.
SEASON ONE
Generally I'd just clean this one up, deal with early-beginning weirdness characterization, and focus more on the horror aspect of the one-off episodes. I think Buffy needed to start off being kind of ridiculous and campy for greater emotional impact later when stuff like Prophecy Girl hits, but part of the beauty of that season is playing with traditional horror tropes and I'd like to focus on that even more.
SEASON TWO
Off the top of my head, the first thing I'd like to alter would be the placement of Go Fish, which feels so out of place. Also I'd cut Inca Mummy Girl entirely because what the ****. Other than that Two is pretty perfect.
SEASON THREE
The only change I can think of is to give Oz and Willow slightly more time before they get back together so it doesn't feel like Willow's forgiveness happens nigh-instantaneously. Other than that this season is so expertly plotted that everything else should stay exactly the same.
SEASON FOUR
Like literally everyone else has said, cut Adam and replace him with Walsh as the main antagonist. I do think that conceptually Adam is interesting and fits with the themes of the season - discovering identity and fluidity of personhood - but there's no reason you can't play with those themes with Walsh too, who's far more charismatic than Adam is and has a greater personal attachment to our heroes. I know everyone hates Riley but I'd keep him the same in this season - he's an important insight into the masculinity issues that plagues Buffy's male heroes, and him/Spike/Xander make up the important trifecta of Boys Buffy Symbolically Neuter.
SEASON FIVE
Five is also pretty expertly plotted, but I would rephrase some of how the Riley stuff functioned. Make him more clearly in-the-wrong rather than trying to present him as behaving entirely rationally and normally the whole time, and his exit being less "The one that got away" and more "****ed up impotent armyboy runs away from his own emasculation."
SEASON SIX
This season's my absolute favorite, but that doesn't make it without issue. I would probably cut Rack and make Willow's addiction to magic, while still being analogous to drug usage... slightly more, uh, subtle. Then, of course, there's the elephant in the room.
I like what Tara's death does for the narrative. Willow's rage is tragic and beautiful and the ending it presents to the season is fantastic. The issue is the usage of the "Dead Lesbian"/"Evil Lesbian" trope, which Seeing Red is an extremely blatant example of. Honestly I'm not sure how to fix this while keeping Willow's presence as the villain intact, but what I might have done back in 4 is make Xander gay and keep Willow in a relationship with Oz. I love Tara/Willow. It's adorable and lovely and serves both of their characters very well. But Willow needed to be gay significantly less than Xander did - it was less relevant to her overarching arc about addiction and control, whereas Xander's masculinity hangups were never properly resolved in a meaningful way and I think making this a result of repressing his sexuality would be a great way to deal with it. It also adds another layer of tragedy - Oz and Willow's relationship would survive so much - multiple bouts of cheating, several apocalypses, and four whole seasons of television - only for him to be killed by a stray bullet, of all things.
I think the Seeing Red AR scene needs to exist. It's the dramatic culmination of 6's hangups regarding consent and sex conflated-with-death. But I'd focus more on Buffy dealing with the trauma of it and less on Spike Feeling Bad About It. And that leads into what I'd change in Season 7 too.
SEASON SEVEN
I like Spike. Nevertheless, less focus on Spike's insanity, more Dawn and Anya, please. Trim the slayers, and focus way more on Amanda, too. And cut Killow, because... it's not good, even if I really do love the imagery of Willow's past coming to haunt her by turning into Warren after a kiss.