Skirts, dresses and kilts are the obvious choices for easy access. Anything tight fit with a zipper feels uncomfortable if you don't lower them down enough and at that point you have to consider if "clothed sex" includes taking them partially off. Swimwear and such may be easy enough to pull aside.
I write a lot of stories about crossdressers/transvestites (because I are one) and cis gender women and most of the sex takes place fully clothed. Love women wearing full makeup dressed in skirt-suits, pantyhose and high heels. There something so erotic about taking a woman (or a transvestite) fully clothed in a power suit: just lifting her skirt, pulling aside her panties, poking a hole in her pantyhose. Often standing up, pressed against the wall especially if there is that element of reluctance.
I'm a little confused by the question as clothing has never been an issue for me. Are you concerned the temperature?
Depends upon the setting. If it is fantasy or some historical period where loincloths were a thing then they are the easiest. If there is nothing underneath then there is only a strip of whatever material to flip aside or slide under. No real hindrance to sex at all.
OK, I'm going to be contrarian for a moment: the best clothes for sex consist of four layers, ankle to chin, and they're all bulky to boot. Make it hard to remove, make it take ages. Lots of belts, lots of buttons, lots of zippers. Because that's paragraphs and paragraphs of potentially good stuff, talking about every inch of skin that's slowly being revealed, about the anticipation building and building and building... All that is great shit that you deny yourself and your readers if you reduce it to "neither of them were wearing underwear so he stuck it right in". P.S. Careful with those zippers, though.
