I feel like it depends on how or when it's asked.
I would assume you've at least met or been with the girl a few times for her to really ask this question seriously?
In which case personally I'd go with honest. Don't use it as an oppertunity to make her jump through hoops (i.e. The "Beauty is common but..." lame-o line..)
The thing is... I think most guys will either just tell her she's "hot" or else for blank for fear of not wanting to just say she's "hot".
Usually I'd tend to give her a compliment on her looks but don't call her hot. Tell her something very specific which you genuinely find appealing. For me, for some reason I've always been drawn to girls with a certain look in their eyes. I can't even explain what it is because I don't understand it or know how to describe it but they have it. Something like that that you are drwn to about them physically.
Mix that in between some things you like about her personality or silly things she does.
I might be mis-interpreting this but I feel this comes up when you are both in an intimate conversation and not when you just met?
In which case, I'd go with honesty, genuine compliments, keep it to 2 or 3, don't use it as an opportunity to gush feeling or go over the top, try to make them unique compliments like something she hasn't heard from regular guys she's dated.
I also wouldn't use it as an opportunity to "neg" her or make her "qualify" herself for the compliment in this setting... I think that would come off badly.
At least that's my opinion... I'll leave the floor open to the gurus
