I have never been able to figure out why but… for me it’s arm pits. It’s not a smell thing which some are into or wanting to have armpit sex. It’s just the look that I find very arousing. Doesn’t matter if shaved or hairy. but going from one to the other (either direction) is way cool to me. The best I can figure is that is a body area regularly exposed in public that is only seen fully with arms raised which is totally at random. A very close second choice would be the abdomen. A short top with exposed mid-section just kills me trying not to be obvious about looking. Any gender with hard perky nipples showing through their shirt is another. Lots of good bits to look at without getting undressed.
If it was a personal body part I would choose nipples even as a male. I would never climax from it but instant turn on buttons that last as long as I want.