I dunno, everything I've ever seen her in that wasn't Buffy the Vampire slayer?
Trust me I'm a huge, huge fan of BTVS, but she's not exactly set Hollywood alight since 2003 has she?
Keanu Reeves was perfect for the roles of Neo and Ted (from Bill & Ted), but is an attrocious actor, I believe the same is true of SMG.
I could be wrong, I'm not a scholar of film or acting, it's just my opinion, and I'd love to be proved wrong by seeing SMG win an oscar for Buffy: The Movie when they eventually get round to doing it.