It's all a matter of personal taste really, isn't it.
There are movies I love purely because of an actors performance, I'll watch anything with William Hurt in it. John Malkovich is another favourite, I loved Dangerous Liaisons and having Glenn Close in it was a bonus.
Edward Norton is brilliant as well.
And because this is a british forum, I love Dame Judy Dench, and no one does sarcasm like Alan Rickman he's amazing. And another brilliant actor is Bill Nighy, The Girl in the Cafe, wonderful.