To answer Chronofox's question: The brazen bull was not used for vitae extraction, but rather to instill terror in other victims. The bellows of the bull carried along pipes into the Nave cells, which caused fear. Fear caused vitae to develop, therefore maximizing overall vitae production.
On Topic: This is only me, but I think the Iron Maiden would be the worst. It may not be as painful, but the factor of controlling how long you have until you die would terrify me. One false slip, one small adjustment, one errant twitch, and metal spikes are impaling every part of your body.
Alexander: Prepare this one.
Me: What, me? What's going on?
Daniel: Um....
Alexander: The Iron Maiden. Bring him there.
Me: ERMAHGERD NERT DA ERERN MAHDERN LERT MAH GER U SERDERSTERC BERSTERDS
If you want a translation,