The problem is, the Major Vampires all occurred during the seasons of the show with a really low SFX budget. For all we know, All Vampires could do Season 5 Angel Wirework all this time. I'm going to say both Angelus and the Master, with a tie for third being Dru and Kakistos, on the grounds we never found out if Dru's powers worked on Buffy, and Kissing Toast, as I recall he's the only Vampire who went toe to toe with two Slayers and didn't die immediately. Well, except for What's my Line, but that was a group battle. I mean, Big guys who become Vampire Henchmen are a problem for Buffy, but, there's no doubt she'll win.