I think I'd rank those two scenes on top too, and I agree that it's the build-up and how they surrender to their instincts that makes the Ian/Cindy scene so good.
I actually think their next scene could be even better. Now that they can't pretend they're not doing anything wrong anymore, that "we shouldn't do this" obstacle would probably be at least as big. In that sense I think their second scene could have many of the same qualities as their first; with the added bonus that Ivy and Jeremy won't interrupt them halfway through.
I also think both the Ian/Cindy and Lena/Holly scenes feel so hot because the attraction isn't only physical. So while the cheating aspect of the first scene between Ian and Cindy in my view adds to its hotness, it might be best to be patient and wait for her to break up with Wade before making the next move. It would still feel forbidden, but it would also allow them to develop their mutual feelings at the same time.
Yeah, sorry to be the bearer of bad news, jduk, but Ivy and Jeremy arrived just in time to stop Ian from getting his release. But maybe it was for the best; I think the interruption was humiliating enough as it was for Cindy. If she'd have to stand there in front of a smiling Ivy and then walk home to Wade – while all the time trying to prevent Ian's cum from trickling down her inner thighs, I fear her shame would make her travel far, far away and never come back.