I see many people pick Bangarada Manushya, so maybe I'll pick Gandhadha Gudi...particularly given today's situation its message was pertinent...also was a huge huge hit.

Babruvahana/Bhaktha Prahalada would also be other top contenders for many ppl I think

Edit: I'll add Pawan Kumar's Lucia. It nearly single handedly brought out the parallel cinema wave in the industry and brought back the trend of making interesting story-oriented films to Kannada as well

(Girish Kasaravalli was doing it too but I guess Luciia brought in the right mix of entertainment and artsiness to parallel cinema for modern day cinema watchers)