"slim". Or svelte (though wonderfully, I never came across this English word before).
You say it when you want to say that someone/somethign is thin, and you want it to sound nice. Its another meaning is somethign along the line or "harmonious" - and it affects its body meaning.
"тощий" - somewhat colloquial, rather negative. "Skinny". Can also be applied to your purse;)
"худой" - neutral style. Sometimes negative, sometimes not. It means more than just "not fat", but it is the most neutral word if you want to sound matter of fact without positive/negative.
"Thin"? But only said about people.
Also in means "bad" (archaic/dialectal version of "bad") : - )
"тонкий" - the most abstract of these words. A pensil can be so.
Can be applied to people too (not often, usually young girls). Also can mean 'fine'.