What is the Hardest faction in the game

[Nebuchadnezzar]

This question is quite simple when you take everything into consideration.

Jihad and crusader factions can exploit the extra movement and extra income and this makes all of them relatively easy factions to play. Further, the catholic factions get income and useful missions from the pope.

That leaves Byzantium and Russia. Russia gets a very large area to control, very low income at start of campaign and the effect of the Mongols.

From the catholic factions it would be (in order) Hungary, Poland, HRE