Synitar is good when there are many tough neutral creatures or the enemy army is larger. He focuses on magic, and if you level up a lucky wizard and have expert luck... in a word: OP. Vaishan is good, Sorghall is decent, and Irbet isn't very good. This is because I've never used him)). Kifrah is good at the start, and extra money never hurts anyone. She also comes with Treasury Management. Minos of the Mine Guardians are good at scouting (if the guardians aren't ranged units).