I'm always a little taken aback by how many fans think Buffy stifled Spike's character. Whilst it's definitely true that Spike's character became extremely Buffy-centric in Season 5-7, he undoubtedly had more development in those seasons then in S2-S4 where he mostly remained unchanged.
Yes, the Spike of S2 is different to the Spike of S4 in the sense he transitioned from a threatening villain to pathetic, neutered vampire. However, those are mostly outside changes rather than actual character development. Whereas in Season 5-7 Buffy was the catalyst for transforming Spike from a soulless evil vampire to an ensouled hero. I think it goes without saying that most of Spike's character development happened within those seasons and that Buffy's character was extremely beneficial to him in this regard.
By the end of S4 it was clear, IMO, that Spike's character was growing stagnant and that the "whacky vampire neighbour" routine couldn't be sustained for much longer. Nor could he remain the villain he was in S2 over 7 seasons. When people say they find AtS Season 5 Spike preferable, it was only made possible coming off the back of his development in S5-S7 and with Buffy. I'm not saying you have to prefer him in S5-S7 but I think it's undeniable he progressed more.