For me it's always about feeling genuinely wanted and like the other person is completely present. When someone is totally focused on you, that emotional connection transforms everything physical.
Communication during — not just beforehand but actually saying what feels good in the moment. Most people are way too silent. It doesn't have to be constant commentary, just honest feedback.
The chemistry. You either have it or you don't and you can't fake it. I've had technically 'perfect' sex that was completely forgettable and messy imperfect sex that was unforgettable. Chemistry is everything.
Trust. When you feel completely safe with someone you relax in a way that's just not possible otherwise. That's when it gets genuinely great rather than just good.