I think the hardest choice is between the Ifrit and the Nightmare. The Ifrit has a very useful fire shield, but it gets taken down quickly, while the Nightmare has a lot of HP and a terror spell that removes a living opponent from the battle for 3 turns.
I have the same dilemma. I usually choose based on the opponent or use the Ring of Fate :D Only not terror, but fear...
Now I'm also hesitating between the Pikeman and the Ballista. I'm leaning towards the former.
