Hard to choose, but Alpen Festung brought so many good lasting memories.
I remember the routine of flying another player (Stickybomb!) armed with sniper rifle to the barn downhill from the church. I'd run for the grashopper, he'd go for the sniper rifle near one of the tents in the base. I'd wait on the runway, then we'd fly low, tracking the river, under the bridge (with dead jerry paratrooper nearby), dodge enemy AA fire, then land on the little field next to the barn. He'd bail and hide in the barn, then picking off infantry and spotting tanks with binocs. With luck I could turn the plane and take off before jerry could respond. So much fun!
I'd also admit surprise-launching a few unsuspecting friendly tanks/APCs using the 30cal kit bug from the bridge at allied base.
I'd hide in the bushes near the bridge, wait for vehicles and then run, dive and quickly deploy the .30 . For those who didn't know, driving over a manned .30 would mess up the physics and eject the vehicle forwards. There was a hill there shaped just so the vehicle would land in the axis airfield. Even scored a few kills that way.
I got some stern admin warnings for that one.