This one

especially as the Osprey doesn't really have standard engines, props or wings. For starters the engines are cross connected via a shaft running through the wing which also has to accomodate their swivelling.

The Props meanwhile are non-standard in that apart from folding they also have to do most of the clever stuff a helicopter's have to in terms of flapping, feathering and dragging to give the pilot some way of controlling it in the hover.

The X2 meanwhile is basically a co-axial helicopter with a pusher prop, not simple but actually fairly standard levels of complexity for a helicopter.