MTW:

Poland/Early/Expert
HRE/Early/Expert
Russians/High/Expert

All other factions are easy.

For STW the hardest campaigns are probably the Oda and Mori on Expert. Imagawa are also difficult if you choose to abandon Kyushu (easy enough vs only the Shimazu with linear progress and easily defended) and concentrate on Honshu (surrounded by Oda, Takeda, Hojo, Uesugi in the fiercest region on the map).