for auto's it has to be a 1911. better if it's in a hard chrome finish.
runner up, a ruger mark II .22 6" blued
third place, any glock... they're not pretty in the conventional sense, but that "all business" look they have can grow on you.
for revolvers? hmmm. I'll just be blatantly biased and say my taurus 608 4" high-polish stainless/black hogue. I fancied it at first glance, but over the years it's only grown more pretty.
single shot? uh... no idea. didn't know they had any sex appeal.