In season 6 Willow got addicted to dark magic and tried to kill not only Buffy but Giles, Dawn, Anya and Xander. I think Willow betrayed Buffy more than Faith! Because Willow was Buffy's best friend and nearly killed everyone and almost destroyed the world after Tara died and started using black magic.

If you ask me it's a tie! They were both Buffy's friends. I know that Willow was her best friend and Faith was a close friend before she turned evil, but their betrayal was hard for Buffy to forgive and forget!

Willow only turned evil in her grief about Tara's death, so I think Buffy understood that a bit more. Faith turned her back on being a Slayer and also tried to frame Buffy for the first murder.

So true! I also think Buffy and Faith had a crush on each other, which broke Buffy's heart when Faith went evil and betrayed her!

^I think Faith might have been attracted to Buffy, but I don't think it was reciprocated.