Hey Guys, I've been playing Call Of Duty for many years and I've played them on Veteran difficulty. The hardest I've played on veteran difficulty was Call Of Duty World At War. Since Enemies know your exact position, and they throw grenades a whole lot which is ultra annoying.
But Call Of Duty Modern Warfare 3 is not as hard. It's actually easy depending on your skill level. But the only "hard" mission is the last mission since bullets are flying everywhere. So if you want to play for trophies/achievements, try the game on Veteran.
Till Next Time!!