The Bulgarians. Have not played only the Russians.
My style of play is of one sitting, rarely will I go even to 60%. Playing the Bulgars with huge armies creates a big economic challenge. Much like the Hospitaliars. In this way , to create an economic base of operations becomes the game. I also will not attack allies, i must first break the alliance somehow.

My big question is how to get piety for the governors and new troops ?
One can bribe a failed crusade to get quality governors and generals, but how do you raise the piety of the Orthodox provinces ?