The most natural way to say it is "몸이 좋다" (lit. "body is good"). So, "I like fit guys": "(저는) 몸이 좋은 남자를 좋아해요 / 몸이 좋은 남자가 좋아요". A colloquial expression for someone with a nice body is "몸짱". You can use "몸짱" as a stand-alone noun or say "몸짱인 남자" (a guy who is a 몸짱). If you really want to emphasize the "muscular" part, you can use the word "근육질" like "근육질의 남자" (muscular guy). However, this phrase may sound just a little awkward/forced in casual speech. Another way of saying it would be "근육이 발달한(developed) (+ noun, like 남자)", but this sounds more bookish. My recommendation is to just say the most neutral variant "몸이 좋은 남자". This sounds perfectly fine in any situation and gets across the point in the most simple manner.