Well, I also love helicopters and despite ArmA2 not being considered a "hardcore" helicopter sim, ArmA2 allows the player to do what the vast majority of (if not all modern) combat helicopter sims don't allow which is to fly transport/assault helicopters in a realistic combat enviourment. Therefore my favourite SP mission in ArmA2 is this one:
http://www.armaholic.com/page.php?id=14027&highlight=WORKHORSEThis mission is called WORKHORSE where the player is a Chinook pilot on standby where he will receive a mission (in a totally dynamic/random way) and after completing that mission the player will receive another one (also randomly) and so on... The mission will end sucessfully when 5 or 6 (random) missions are completed.
There are lots of missions that can randomly spawn to the player that ranges from Troop insertions, troop extrations, evacs, cargo lift and transport, etc...