I have several favorites.
The Firbolg from Awaken Sleeper saved my bacon in my EA R'Lyeh SP game I finished recently. Fairly tough with decent leadership and the standard ability really made them shine for me.
Dai Onis are just cool! Big and nasty with a good amount of magic and able to throw fire and get dirty in melee, on top of having to be 'killed' twice make these guys rock. Being a demon for anti-demon spells, and the gluttony sting a little (I am not overly concerned about the -research since I don't make these guys for reading
) , but these issues are minor to me compared to their value.
Mandahas from the Lanka national spell. These guys served me well. Similiar to my Dai Oni comments above and even worse gluttons, and have that nasty fire vulnerability, but so good in so many ways.
Demon Knights also have rocked for me. High protection and morale combined with a hard-hitting punch. I prefer Infernal Crusade to get a bunch at once over the single summons of course.
Vastness'. All I can say is:Buwahahaha!