you don`t get how "i don`t care how brilliant she is" phrase is so deeply important to me cause spike is practically the only character through out the show who openly establishes and admits, to his own displeasure, that buffy summers is a really smart and bright person, who constantly one ups him and leaves him completely astonished by the schemes she so easily pulls off. willow maybe is another character who gets quite close to praising buffy`s intelligence, but she often sounds almost suprised by it, and we know that she likes being the superior one brains wise anyway. but to spike theres no surprise at all, he studied her closely, he sees her, he knows that a quick mind is needed for fighting unpredictable mosnters and you have to think on the spot, mid blow what your next 3 moves should be to not only gain control over the situation, but to use the space for your own advantage and defeat the opponent - and any other person would simply crumble under such pressure (we see everytime with the scoobies when they try to do buffy`s job).
buffy summers IS smart, she IS brilliant, she is the one who comes up with the most successful plans, she is the one on whom people rely (c`mon, wwbd exists because of that), she is able to simultaneously fight and come up with the wittiest and funniest puns - and you need to be quite clever to be this sharp and quick-witted. i wouldn`t even say that she is not booksmart but a street smart, cause under the right circumstances and with the right professors on her favourite topics she has a potential to be willow`s level book smart: we saw it when she got a pretty big score on her final exam at school, she loves poetry and clearly fascinated with language (william, heeeey~)we saw it in wild at hearts when professor walsh wanted her to lead the discussion, willow and buffy even had a bit of debate. she can be it, she just constantly gets discouraged by people in power around her cause they don`t want to give her the benefit of the doubt, cause they only see her as trouble, cause she gets easily distracted, cause they all are prejudiced as hell and don`t want to be proven wrong.
and i think buffy is quite used to such treatment which truly breaks my heart. so to one person to recognise her brillance despite all those obstacles is so incredibly important and touching because he sees her without that judgement and he doesn`t even know the impact he has with that vision of her. and even though he doesn`t say it directly to her and this scene follows by a funny transition (which i also like!), but it is so significant that somebody finally utters such words towards buffy.