Level 1 - Warriors (Wall of Shields - awesome)
Level 2 - Spear Masters (Chance to stun - better than pulling (many say you can push them into walls of fire, etc., but I don't think situations arise that often where units are crowded in one cell next to a wall))
Level 3 - I don't build them (in my opinion, both abilities are stupid; you run to block archers, and they, on the contrary, are pushed back)
Level 4 - depending on the situation (depending on who I'm playing against, against fast factions - Berserkers)
Level 5 - I buy them in equal numbers or only one type, but I divide them into 2 stacks (one places a fire mark, the second casts a spell)
Level 6 - Yarls (against slow units - Erls)
Level 7 - Arcat Dragons (well, it's obvious)