I think, if we're talking training and technique and what it means to be a Slayer and not Buffy Summers, then Kennedy and Vi actually come out top for me.
Like [MENTION=11853]Jodast[/MENTION] says, Buffy is only as good as she is because of her friends. If every Slayer in history had friends like Buffy did, then they all would have been as good as Buffy. (Of course, not every Slayer faced apocalypses like Buffy did)
Kennedy and Vi have been in training since they were kids. Kennedy says she's known how to use a crossbow since she was eight. Faith and Buffy never had that. They were thrown in at the deep end and while Buffy learned how to swim pretty quickly, Faith became overwhelmed and ultimately almost drowned until how to float.
If she hadn't been killed so quickly, I think Kendra would have been my top for natural ability and having the smarts to know what being a Slayer - a 'real' Slayer - means. She was overcome by Dru's hypnosis, and I don't blame her for that, but it was her cockiness that led her into Dru's thrall, thinking she could beat her.
So, yeah, I think Kennedy and Vi are my two on the 'best' person capable of being a Slayer. They were raised and trained for nothing more. And while I don't like Kennedy and I'm starting to think that there's some sort of Supernatural crossover going on where Vi is undercover as Charlie Bradbury, lol, I think they knew more of what to expect when they were going into the whole 'Slayer' thing than any other Slayer who came before them.
And being prepared is half the battle.