what are you really concerned with ?
-occlusion ?
-distance lod ?
-streaming ?
-performance ?
-memory footprint ?
-dynamic lighting ?
-dynamic collision ... can i blow up a house ?!?
those are importent things to think about when choosing your engine.
if you dont need dynamic lighting than source and unreal move up in the ranking but if you do cryengine and unity move up instead than again if you need proper streaming unity moves down etc ...
give us a bit more info on what you need and what you could sacrafice