I'll list it by role:

Tank: WAR

Healer: WHM

DPS: BLM

As someone with an RSI I main WAR and RDM pretty comfortably. Meanwhile stuff like MCH and GNB feel uncomfortable to play. I think with stuff like this, RDM and WAR replace buttons based on context which makes it easier to map, which means less gymnastics with my hands, which means less pain and discomfort. I made the investment of getting a good MMO mouse where I can transfer some of the complexity of the keyboard to my mouse, which I found helped reduce the strain on my keyboard hand.