Mine are...in no particular order
Plaguelords...amazingly fun race. My guy is a Plaguelord Archmage. I love the looks of the buildings and towers, too. And that Fire spore is really powerful. More of an offensive style race, get a lot of damage+ research.
Dark Dwarves is another fun one. I really enjoy a Dark Dwarf Tinker, get more powerful buildings and what not and I made my guy a fighter. Kind of a slow race,more defensive.
Fey are a funny race. Its hilarious to see tiny people attacking dinosaurs or other giant enemies. The voices are fun too :D I made mine a Fey merchant, not the most exciting hero, but I get cheap units.
And runner ups go with Undead, but I never play one, seems too common. Dwarf is fun, but I prefer Dark Dwarves. And I like Swarm, mostly because I love insects and it has...ANTS! :D. Haven't found a hero I liked though, for Swarm.