Buffy and Riley. I knew when I was supposed to see it, I knew all the points where I was supposed to think they were sizzling, because the writers spotlighted and highlighted it with giant neon arrows…but it never convinced me. Maybe because back in those days, before online fandom and meta and fic, I was still supposed to think that Buffy loved Angel? And I couldn't see her moving on so fast? I don't know. But I think it was more because Riley always acted like he knew more than Buffy. Like, once he decided he wanted to be with her, her objections and reservations didn't matter. He handwaved her past trauma away as if it didn't matter because he would be different, better. Whatever. So I mostly got impatient with their angst - such as it was, and their slo-mo first love scene didn't do anything for me on any level. I'm glad Buffy finally got to have some consequence-free nookie, and they seemed to have a very healthy sexual relationship, but none of that turned me into a voyeur. And if I don't have pervy thougths and want to watch like a Peeping Tom outside the window? It's not gonna work for me.
Runner up goes to Lilah/Wesley. Never saw it, despite most of fandom being very enthusiastic. I think it might be because I never found Wesley personally attractive - he's all wrists and head to me - so seeing him in a love scene left me cold. There was one episode where they had just clearly finished having a very good orgasm, and they were both sweaty and she was lying on him and all I could think of was "ewww, take a shower."
Or maybe because in a small back corner of my non-C/A-shipping mind, I always saw him with Faith. I liked that dynamic, the one where Wesley was stronger mentally and emotionally and could carry Faith like a sleeping child, even though she could break him in half without breaking a sweat, even though when they throw themselves out of a window, Faith is the one who will take the brunt of that fall because she could, and he'd let her without thinking twice about it. I like that flip in traditional male/female roles more than I like Lilah's Lilith tempting Watcher Wesley. It was cliched and tiresome.