I like my GoR-ed Tarrasque, empowered to S4 so she can Gateway her buddies in Team Tarrasque all over the map.
The original question is a bit off, though. As Edratman wrote, it's really hard to pin down the "best" unit in a game this complex. Best at what? Best thug? Best mage? Best support unit? And even if you manage to narrow down your terms, you'll find that someone has a counter-tactic that will leave your uber-unit crying for mercy.
So, yeah, there is no "best" unit in Dom3. Certainly, any of the unique summons are very powerful. For example, the King of Banefires makes a fantastic supercombatant. Air Queens can use the Cloud Trapeeze - Staff of Storms - Wrathful Skies trick to wreck entire armies. Arch Devils can lead a powerful raiding force behind enemy lines. Etcetera....
In a different line of reasoning, I also like units that are reasonably powerful, versatile, and easily available. According to that standard, I like the Barbarian Heavy Horseman from Late-Age TC. Those guys are great! You can buy them in any fort, they have a strategic move of 2 (yay, horses!), and they'll grind up nearly any mundane army into dust. Just set them to "Hold & Attack (whatever)": they'll send three volleys of composite-bow arrows at the closest enemy, which will then be followed by a devastating charge. They have good defense and adequate armor, so they're reasonably survivable. Yeah, I'm a cavalry fanatic.