Ulm is probably the most straight-forward nation. It has really tough troops but it won't benefit much from the awesome bless of the prime pretender. Late-game power often comes from forging items for thugs/super-combatants.
Pythium has very solid troops and good mages, but you'd need to learn how to set up communions and storms for mid/late game power.
Tien'Chi has solid troops. Their mages have a lot of different paths, though, which is great for experimenting but difficult to put together a coherent strategy.
Pangea, decent troops, earth and nature mages which can be simple to use, though most of their late-game power comes from tapping into blood magic.
Of those options, I guess my top one is Pythium. Communions are probably easier to learn than blood magic, equipping thugs, or whatever it is late-game Tien Chi players do (lol).
