On my Nikon FF bodies, it has to be my Tamron 150-600 f/5-6.3 Di VC USD. No zoom this relatively cheap has the right to be so good, and impressively tack sharp at 600mm and wide open. it's a giggle.
For my M4/3 system, although I have some really nice lenses, the one I always come back to is my Olympus 12-40 F2.8 Pro. It's a belter !