I like to think things in a very evolutionarily sound manner - whatever is the most efficient and effective body type.
A ridiculous amount of muscle, if not highly functional, easy to carry around, and practical, is only slightly more useful than it's weight in fat. It takes up energy and puts stress on your body.
Having very little muscle hurts you in every way imaginable. Your functionality necessarily increases, and you become incredibly more susceptible to external stressors.
Long story short: Get a lot of muscle, make that muscle work well for you.
For guys I like considerable muscularity. I am not into skinny boys or chubby ones. I want someone who, just by looking at them, I can tell they eat well and work out regularly.
For girls, I have to say I love a variety, but more important than anything is an overall "solidness". Even tiny women - five feet tall! - can be "solid" and hard. My aunt weighs 98 pounds at 5'0" and a friend of mine weighed 120 at 5'2". The difference? My aunt sometimes looks like she's recovering from a serious disease, and my friend is compact but well-defined.