Crusades easy? If you play one of the Crusader states, maybe. Byzantium and especially the Turks are a real challenge.
I think the Teutonic campaign is my favourite, recently I conquered that whole map with Denmark. Every faction has good options available to it, it's well balanced on the campaign map and with most factions internally, and the atmosphere is just superb.
The Americas are okay, but you can really only play with Spain. I wonder if there are some mods available for it which would allow you to play as England or France, should open up some more options. As far as the natives go, I only find the Apachean tribes fun to play, I really miss having cavalry with the Aztecs or Mayans.