I don't think any particular activity is the ultimate act of submission. I think it is when the sub truly abhors or is scared of the idea of doing something but does it anyways through their own acceptance of the Dommes will rather than being forced.

I have been in this position five times in my life with four different Dommes and it all had to do with the circumstances rather than the activity itself. Three of the activities were on my hard limits list and that did play a part but under normal circumstances I would have just refused those things (and had in the past and since). However, in those particular moments the Domme's will overwhelmed my own and I submitted.

Don't get me wrong, being physically forced past your limits is extremely powerful but I think being overwhelmed by Her will and just giving in is the "ultimate act of submission" .

Asp