Masteries on Heimerdinger?

I would argue for stormraiders surge if you go up the cunning tree or deathfire if you go up the ferocity tree. Yea, I know heim is all aoe spells, but its still a 10% AP increase (still decent). Go surge because it allows you to run and reposition quickly if you get jumped on.

There isn't really a good mastery on him, but I'm sorta okay with that considering how passive and consistent a champion he is.

Since CDR does effect his turret recharge rate, I would consider getting the mastery that increases the CDR cap to 45%.

And since we have to go that far down into the tree, then I would select either Thunderlord Decree for the shock effect, or Stormraider Surge for the movement speed when your turret/ult turret deals enough damage to give the speed boost to escape someone trying to dive you.