As a general rule I like to use as few regional units as possible. If I want to play as a country, in my mind I will use their units 95% of the time. Regionals are for emergencies and for garrison if they are cheap enough.

Mercs on the otherhand come in very handy but I never keep them around for long enough.

Admittedly there are some very powerful and useful regionals out there but its not worth the effort for me to find them using recruitment viewer. Any off the top of your head that are a must have if I have a faction in whatever part of the world?