Something RMR sized in whichever flavor you like. I've got the Vortex Venom with 3 moa dot for my gen5 G34, but have not mounted or used it yet.
I believe Weigand makes bolt on mounts for 'em.
ETA- I've been thinking about putting a red dot on my 5 1/2" .41 Blackhawk. My eyes need all the help they can get.