[curtadams]

I'd recommend middle age Man or Marignon for learning. They field good versions of the basic Dominions army, which is infantry in front of archers, with flanking cavalry to sweep up stragglers. Both have access to a decent magic set, with army enhancers, magic items, and summons, although the mix varies, so you get to play around with magic without having to know how to use it. Ulm is mostly a good start but IMO the magic is too bland apart from items, which requires good supercombant/thug design to use.

Vanheim has no archers but their high-defense infantry and cavalry are incredibly strong so it's also a good nation to learn on. It's a good nation to learn thug construction because the glamourous commanders are so strong.

Some trickier things to avoid:
Trampler nations: Arco, Patala
Blood nations: Mictlan early or late, Marignon late
Funky Dominions: Ermor or late R'lyeh
Cavalry nations: late Tien Chi

Many of these are quite strong but using their strength requires special tricks with magic or army layout.