I voted for Franklin I or II.
It seems less impressive in retrospect, but at the time Rich was at the top. Everyone knew Silva was good, but people really didn't grasp just how large the disparity in skill actually was. He absolutely destroyed Franklin, one of the best fighters the UFC had ever seen, in a display of domination that had extremely rarely been seen at that point. It was shocking to see a guy enter the UFC, and completely trash a champion in only his second fight in the organization.
My second vote would be Sonnen I. Most men would have been psychologically defeated, completely crushed when that 5th round came about. But Silva was wise and crafty enough to keep just something in reserve, and was smart and calm enough to throw his last ditch attempt at precisely the right time. Sonnen was open for a triangle for a long time; but if he'd thrown the sub at the wrong moment and Sonnen defended, then he would have lost the element of surprise. Instead he waited patiently until almost the very end of the fight, and pulled out a preposterous victory. That is what champs are made of.
Henderson was a great win, but I feel it is more impressive in retrospect. Henderson has done well since then, brutally KOing Bisping and grinding out a horrific decision against Shogun. On paper Henderson was a bad match-up for Silva, but in reality it didn't happen that way. I don't think a rematch would go any better, I think its just a bad match-up for Dan's style.
The win against Forrest was impressive, but Forrest is a very sloppy striker, and so isn't that impressive for me.