My definition of power is when these guys hit you, you go to sleep or have to cover up to recover.

There is no major statistical breakdown or evidence just my opinion based on what I've seen

No distinct ranking. I based it on instances of striking power (I apologize if I missed any):

These are just the fighters, I believe have striking power and examples of it being used. Only included fights against UFC fighters unless they were notable in the UFC, Strikeforce or Pride like Tim Sylvia, Arlovski, Rizzo (mostly for Fedor), Fujita etc etc

Heavyweights:

Fedor Emelianenko: Sylvia, Arlovski, Rogers, Rizzo, Henderson

Junior dos Santos: Werdum, Struve, Nelson, Mir, Gonzaga, Carwin, Yvel, Velasquez

Shane Carwin: Gonzaga, Lesnar, Wain, Wellisch, Mir

Lavar Johnson: Barry, Beltran

Pat Barry: Morecraft, Kongo, Cro Cop, Hardonk, Eversen

Travis Browne: Struve, Mcsweeny, Griggs

Alistair Overeem: Duffee, Lesnar, Fujita,(Werdum flopping is a testament to Overeem's power), K-1

Brock Lesnar: Couture, Herring, Mir 2

Roy Nelson: Struve, Herman, Schaub, Cro Cop

Andrei Arlovski: Rothwell, Nelson, Buentello, Matyushenko, Freeman, Sylvia1,2,4

Mark Hunt: Tuscherer, Wanderlei, Cro Cop, NIshijima, K-1

Cro Cop: Barry, breaking Bob Sapp's eye socket, headkicks leg kicks body kicks

Cheick Kongo: Pat Barry, Cain Velasquez, the ground and pound one I think it was on Al-Turk

Matt Mitrione: Marcus Jones, Hague, Morecraft,