Anything with good mil. ideas. Stuff like attrition/fort defense helps too.
So, for Europe:
Obviously Brandenburg, Teutons, Sweden, Finland (release from Sweden), Smolensk (release from Lithuania), Poland and Muscovy (if you somehow manage to make a march off them), Scotland (good built-in quantity bonuses, above-average generals), perhaps someone else i've forgotten.
Steppes/Middle East/Persian region: Basically every reformed nation with Horde ideas, Rassids, Ardabil (its first idea, along with +10 morale, gurantees it will always has full Mysticism with its additional 10% morale and extra Fort Defense), Aq Qoyunlu, Transoxiana (decently hard to make it a march), Luristan, Mushasha (same reason as Ardabil+good for converting religion)
India: Nepal, Mewar (or anything with Rajput ideas), Andhra (because great Fort bonuses to stall your enemies), Vijay (large nation = hard to march), Nations with Indian Sultanate ideas (great for religious stability along with some military bonuses)
East-Asia: Japanese Daimyos (specifically Uesugi, Shimazu, Oda), Dai Viet (great defensive bonuses+decent military), reformed Manchu-idea nations.
Africa (heads up to
@Horn and Ivory): Air, Songhai, Benin, Butua, Imerina
Edit: Added Africa and Mushasha
