Silva? (Leben, Franklin x2, Sonnen, Bonnar. +3 pre-UFC fights)
Aldo has a ton including his WEC and pre-WEC fights 
Aldo has one UFC win with a knee (Mendes) and two WEC wins using knees (Swanson, Perez).. Only three in his entire career.
In terms of the actual knock out blow? Yea Silva must be the most in the UFC.
Overeem must be up there if you include his K1 fights and pre-UFC fights as well.
You can add Ben Saunders
Rousey. Definitely used them to get into the UFC atleast. 
I know not all mma fights, but he arguably has the most devastating knees in mma, with the exception of maybe Anderson.
Tyson86 - In terms of the actual knock out blow? Yea Silva must be the most in the UFC.
Overeem must be up there if you include his K1 fights and pre-UFC fights as well.
Well.. let's stick at least stick with the sport of MMA. But yeah, Overeem has a bunch of MMA wins via knees dating back to 1999.. (11 wins)
Overeem has ONE win via knee in the UFC.
Bucephalus - Overeem has ONE win via knee in the UFC.
Ok so the question is specifically in UFC fights alone? Then it must be unarguably Anderson.
harr3929 - Rousey. Definitely used them to get into the UFC atleast.
Ohhhhhhhhhhhhhhhhhhhhh
Maybe not wins in the UFC but I think Yves Edwards is probably up there.