I've been a Rachel Weisz fan since I saw Enemy at the gates in theaters ten years ago (not a great movie, but a great cast at least). She has a very singular beauty, not to everyone's taste, but her acting is so powerful... In The Fountain, she's the most beautiful woman in the world (partly thanks to Aronosfky's masterpiece, of course...). She was stunning in one of last year's greatests movies, Agora, lovely in the underestimated Brothers Bloom... in short, I watched Definitely maybe for her and her only.
So, I was quite surprised when I realized that I was more impressed by Isla Fisher's performance. Of course, her part is better, her character's the most important next to Ryan Reynold's, and the movie is good, so it makes us wanna root for her from the beginning, but I was still surprised. She's irresistible in it, and very well written as a character.
Aside from that, Elizabeth Banks character is quite boring/insignificant (that's one of the few movie's flaws), it doesn't emphasise her beauty, to which I'm not really sensitive anyway, so... I'd say 1) Isla Fisher, 2) Rachel Weisz and 3) Elizabeth Banks.
I am also a straight woman/girl (I can't say woman yet its to scary lol) i think Isla fisher was the prettiest, Rachel wiesz is too but I didn't like her charactor.
I would have to say
1. Isla Fisher
2. Rachel Weisz
3. Elizabeth banks
Agree with the Elizabeth banks comment, she is beautiful but in this that wasn't emphasized enough and her character was awful what she did to Ryan Reynolds, which is also another turn off