I'm going to go off slightly on a tangent but for me it's kissing that's the best. It's the psychology of it, especially if it's someone you haven't kissed before - it's the introduction, the lead in, the anticipation of all that may follow that plays in your head as you kiss...? Nothing more erotic than kissing because of that.
This! This, so much this.....if he/ she doesn't do it for me in the kissing department, it's unlikely to go further...
GG x