Bosch Duraterm, usually get mine form ebay or the local Bosch dealer.
Other brands are cheaper, but you get what you pay for, last set of plugs lasted all but 2 years, ditched them for a new set when i had some starting problems which turned out to be an air leak so the plugs would have gone longer than that.
I'm running on veg and these started me all through the icy winters we've had.