Compare/Contrast
I assume that everyone knows that Angel is a vampire in the series (I didn't, so I suppose that isn't a safe assumption to make) and that Buffy is, as the title would indicate, a vampire slayer. In Twilight, Edward is an glittery immortal with a blood fetish, and Bella is a weak-willed girl who apparently smells better than a Thanksgiving turkey.
So in Buffy, you have this wonderful dynamic between two strong characters who are destined not to get together because their goals in life are to kill each other. In order to resist the urge to do so, they must also resist the urge to eat each other's faces. Edward wants to eat Bella, and Bella is all for it.
Bella and Buffy share the young age of 16 (as well as five letter names starting with "B"). Edward and Angel are both pedophiles (and stalkers).
Bella is all for dying and vampirism. Buffy is fighting for her life while trying to rid the world of evil bloodsuckers.
Edward and Angel both were horrible bloodsucking monsters that killed a ton of people, but felt bad for it later.
Angel is attractive with confidence and drive. Edward is mopey and sparkles.
Buffy obviously came first, but more than that, Buffy was done better. Sure, she's ditzy and quite obviously 16, but her character grows and changes. As she comes to accept her destiny in life, she pushes to be a better and better person. She learns things. Bella is a stagnate shell of a human being, moving determinedly forward to her stupid life-ending goal.
Verdict
Twilight is a cheap ripoff of Buffy, taking the main character from being a strong independent woman in love with the object of her torment into a shell of a human being that goes after a man who should know better.
