Left side.
I always want my right hand free for the rear brake. And my left thumb is more flexible. The right is stiffer from injuries and arthritis.
And I'm adding one of these soon for riding the MUP. Pretty much hands-free, other than to activate/deactivate it. It rings continually while riding, varying with the terrain. Perfect for busy days on the MUP. Unless everybody else gets one. Then it'll sound like an ice cream truck flash mob.