On-screen chemistry is often so tricky because it has so many different components and it needs both actors to be completely on the same page in terms of everything from the littlest expression to the timbre of how they say a word. A glace, a quirk of the lip or eyebrow will be replayed over and over by some but in the end its really all about the scene the fan/shipper/casual viewer thinks about when they think of the two characters together; does that chemistry move you?
With Buffy and Angel I think of scenes from Angel, Reptile Boy, What's My Line, Bad Eggs, Becoming Part 2, Amends Helpless, and I Will Remember You; these run go from sweet romance to repressed sexuality to...some sweet back and forths to well, heartbreak. Lots of heartbreak. I'm sure there are some romantic scenes from other episodes that don't impress me so I don't remember them. But these? These are more than enough to convince me that this couple are the coupleist couple that ever wanted a happily ever after. They show it, I believe it, so I can acknowledge it. Simple.
With Buffy and Spike I have much more of the antagonist vibe material to work with; which to some might seem more 'real' (or hell, even sexy!) but to me that is not 'shipper chemistry' per se. I wouldn't ship Buffy and Xander just because they have lots of heartfelt arguments; you need those softer moments, the moments you see the real couple and their dynamic.
Up until S6 Spuffy couple chemistry consisted of Something Blue, which is played as ridiculous broad comedy. Buffy has never been sappier or more immature in love than she was in this particular episode. And its just... funny, its not like a 'wow, these two are a believable couple' thing; the idea is ridiculous.
S6 is its own thing and I believe that their is sexual attraction and whole bunch of messed up emotions from Once More With Feeling onward. But certainly nothing strong enough to survive the AR in my estimation.
But in S7 all I think of is their awkward scene in End Of Days, the one where he walks in and finds her back at her house, then they have this weird, claustrophobic talk that's full of bad camera angles, whispered bad jokes and pregnant pauses; this is their most significant back-and-forth conversation about their actual feelings and it completely fizzles. It just feels like Buffy wants to show some gratitude to Spike but also desperately wants out of the conversation and get some head space (in contrast she is immediately relaxed and chill with Angel until the idea of Angel staying crops up).
Most of the other Spuffy stuff after (apart from the ILY/NYD) is just non-verbal 'significant staring', the most subjective of shipper stuff; hell its what us non-shippers base half our 'evidence' on lol.
I'm not trying to convince anyone one way or the other because when it comes to shipping chemistry its just as much about what you want to see as to what is actually there. And some ships are just written ambiguously.
