Well, I prefer to play my own gender, so for females I'll give examples of what I prefer to play as, and for males I'll give what I think looks best:
Females: Body types 2 & 4, depending. Body type 2 is more non-combat-oriented to me, so that's what my Sorc/Operative/Smuggler use, and 4 for my BH and both Warriors. I know body type 3 was intended to be the 'more muscular' version, but the bone structure on that just feels WAY too wrong. (I wish it didn't feel so awkward, as I'd love to use it!) Body type 1 is pre-teen, anorexic, or super model (though anorexia usually fits that category as well). If I ever got an idea to play a 'kid' character, I'd choose 1, but it's doubtful I'd do that. I'm really glad to have all the choices, though!
Males: Body type 2 in general, with 3 sometimes. Body type 1 is, again, teenager/kid. Body type 4 is morbidly obese or some cartoon artist's ridiculous imagination of what 'upper body strength' means. (I think a general 'fat' guy would work far better than the body shape they chose, but, hey, it's obviously popular to some due to how often I see it.) However, I'm not comfortable gender-bending even in virtual environments, so my preferences make little difference here.