The easiest event to earn a platinum medal that I've found is the 4th event in Tours of Motorsport which is Raptor Strike. You drive a Ford Raptor pickup truck and knock down bowling pins. If you turn off all of the assists and drive quickly you can easily complete all three bonus objectives and get a platinum medal.
You can also repeat this event over and over for the Top Rated Driver achievement for earning 50,000,000 race points.