I don't think Keanu Reeves is much of an actor -- but he's a good movie star. He has a limited range, and as long as he sticks to it, he's golden. I liked him in The Matrix, Speed, and Point Blank; in Much Ado About Nothing, not so much. He was out of his element.
Rachel Weisz, I agree, is drop-dead gorgeous, smart, and talented enough to play an ugly and dumb character convincingly. I haven't seen Constantine and don't know Peter Stormare, so I can't comment on that.
I do like Keanu Reeves, though, as you say, he is limited. But I think he chooses movies that are comfortable for him.