Kerala. And it’s not even close. In some respect it’s strange how different it is from rest of us.
It differs from all three major ethnicities in almost every sphere- religion, dance, cuisine, Gods, cousin-marriage, festivals…I will explain a few here
Cousin-Marriage: Except Kerala all other states have very high rate of cousin marriage and in fact it the most preferred among Kannadigas, Telugus and Tamils.
Cuisine: The fact that beef eating is acceptable in Kerala is unique among all the Hindus.
Religion: Kerala is religiously diverse while all other states in South are much more homogeneous. No other state in India is as diverse as Kerala in religion.
Dance: Their classical dance forms Kathakali and Mohiniyattam are easily distinguishable.
Festivals: Kerala doesn’t celebrate Deepavali. It doesn’t celebrate Ganesh Chaturthi. It’s major festivals are different with a totally different mythology.
Language: Urdu usage is near zero among Kerala Muslims- this is unique- not even Tamil Nadu has this much uniformity.