I'm a huge fan of the armies of Rohan and Fallen Realms. Rohan is great because it fields plenty of cavalry and no one can say you're using a 'broken' combo or special rule.
Also it gives you a good idea of how skilled you're becoming as it has a steep learning curve. I like Fallen Realms because I enjoy playing an evil army not made up of Orcs, Goblins or Uruk-hai. In many ways, Fallen Realms is also a highly versatile army with the ability to field the heavy infantry of the Easterlings, the deadly archers of Harad, the dreaded Morgul Knights and the elephantine Mumakil.
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