I would prefer arms to be buff because it's nice to hold on to and it feels cool. Pecs would be my least liked because I had an ex that was jacked but I didn't like cuddling with him because his pecs were to hard to use as a pillow.
I think Deltoids are important to tone though, as nobody wants to have bony shoulders. I wan't to get bigger Latissimus Dorsi and Obliques (upper and lower) so my ribs are less apparent, and so I can get more of a V shaped torso going on. Personally, I just work my whole body at the same time, so I don't end up disproportional. I don't really work out that often though. I end up running more than lifting. I am taking weight lifting next year though.
What you mean "what muscles"? You gotta have all. Chicks won't dig some guy with just some big arms and rest of em small. No, popye is just show and they won't like dudes with just bigger chest and other things small nor skinny guys with abs since its like tits on fat chicks, doesn't count