1) Cerberuses - they are very strong, unbeatable, and can kill 3 enemies at once.
2) Ifrits - simply powerful, with a fire shield and deadly speed. And if they also have Armageddon, they are even better!
3) Pit Lords, demonology. You can raise horned creatures to 1000+ (on XL maps).
At the start, there are 3 hero options:
1) Octavia, because of the 350 coins per day.
2) Ignat, because of the imps, and you can build a Golem and go kill small mobs.
3) Rashka, with Ifrits.