The actor who instantly popped into my head when I saw this question was this man:
Ramesh Aravind
Considering the kind of roles he's played and movies he's choosen to act in, he definitely deserves more appreciation. Overshadowed by his contemporaries, this gentleman actor is one of those few underrated kannada actors with a limited fan base.
but, considering he's well known and regarded highly by all Kannadigas, I wouldn't add him to the list, lets move on,
the second person that came to my mind was this guy:
Achyuth Kumar
although famous after he played roles in Lucia, Drushya and Ulidavaru Kandante, this guy has been around from a long time and is one solid actor who can pull off pretty much any kind of character/role with ease.
(I remember him from his early days of acting in one of those serials on ETV kannada, the name of which I can't recall at the moment.)
and then this guy,
Ram Kumar
With good acting skills and decent looks (oh wait, better than most of his contemporaries, actually) this guy should've gone places. But he didn't. Still wonder why. underrated!
I would also add Srinivas Murthy and Daisy Bopanna to the list.
I have left out a few others as other answers already mention them.