The sight of a beautiful woman looking me right in the eye while she plays with herself.
Years ago I was with a regular, she is straddling my thighs playing with my dick while we chatted. At this point I am mostly hard. Then she asked me what turned out to be a sensitive question (but completely innocent from her standpoint) and my dick went down fast
Realizing she had asked the wrong question she lay beside me while I stared straight ahead getting my feelings back under control. About 3 - 5 min later I heard a soft sound, I turned to look at her and she is staring straight at me, one hand between her legs and the other squeezing a breast. I got so hard, so fast my cock might have gotten whiplash if it were possible.
Ever since then that has become a huge turn on.