Most of Seasons 1 & 2, I don't care to rewatch bc I think they both have a lot of shallow-ish filler episodes. The only ones that I really like to rewatch are Angel, Prophecy Girl, When She Was Bad, Halloween, Surprise, Innocence, Lie to Me, Passion, Bewitched Bothered & Bewildered, What's My Line 2, I Only Have Eyes for You & Becoming. Sometimes I'll be in the mood for my cheesy monster of the week guilty pleasures Witch, Go Fish, The Pack, The Puppet Show & Ted.
NONE of Season 3 is skippable in my opinion. It is the flawless season.
Season 4, depends on my mood like I hate Wild at Heart but I love to hate Veruca and her temptation of Oz is intriguing as they are both wolves and it's a very cool animalistic depiction of cheating which is a realistic fact of life and human nature sometimes for better or worse.
As much as I defend Where the Wild Things Are, I know it's a skippable episode. It's definitely underrated as far as scary episodes go though because the humor and sexual element overpowers the horror but there are a couple really creepy moments (Willow finding the ghost in bathroom and the ghost girl running through Anya). And Giles singing in the coffee shop as well as Spike & Anya in the Bronze makes it worth watching for me. I love Season 4 and find it very rewatchable because it has such a light, accessible atmosphere- more than any other season,
Season 5 I find hugely overrated overall even though it's visually beautifully shot (the show looks the most rich in this season) and I love how they introduced Dawn, I really only enjoy Buffy vs. Dracula, Real Me, Family, Crush, Tough Love & Spiral to rewatch. Everything else is skippable to me most of the time. Maybe I'll watch Forever if I feel like crying. It's perhaps the best constructed season start to finish but that doesn't mean it's the most enjoyable, interesting or rewatchable. I do appreciate though that it's the best season for couples Willow/Tara and Xander/Anya.
I love Season 6, probably right after seasons 3 & 4 but Hells Bells & Seeing Red are the worst. Seeing Red is great to watch reactors freak out to though when Warren kills Tara. I also appreciate what Normal Again does (Buffy chooses to be a hero even if it is only in her mind, leaves open possibility of the alternate reality) but I don't care to rewatch it really. Every other episode I love to rewatch this season even if they're are sick and twisted moments, that's what makes this season so entertaining; I love the way it depicts serious real life issues. Aside from the scene with Giles confronting Willow about resurrecting Buffy, Flooded is pretty skippable too though.
Season 7 is weird. I think I love and hate it equal amounts. Lessons is not a bad opening but I don't like to rewatch it really. I will also usually skip Beneath You, Bring on the Night, Showtime, Potential & Touched. I hate Lies My Parents Told Me (even though I love any Drusilla screen time) so can do w/o that. And even though I loathe Andrew, I love the documentary-style of Storyteller...and mostly just the scenes where Spike is acting for the camera, Xander & Anya have their convo on the couch w/Andrew interviewing them (then rewatching it and mouthing the words) and Andrew describing the characters of Buffy, Spike, Anya, Faith etc.
Angel is a whole different beast. I appreciate all the dark twisted drama (especially in Seasons 4 & 5), even if it does piss me off what Joss did to Cordelia just because Charisma got pregnant in S4; It's so stupid, as if they couldn't just hide her pregnant belly. I usually skip most of Season 1 though except for the Faith episodes and I don't care for Season 3 really whatsoever except for a few episodes (Lullaby, Forgiving, Birthday, Double or Nothing (underrated imo, maybe my favorite Gunn-centric ep) & The Price (one of my favorite monsters of the week). I love Season 4 though, every episode except for Awakening & Sacrifice. S5, I skip Cautiounary Tale & Why We Fight.