This bad boy right here.

It's a bucket trowel, essentially a brick/block trowel that's been cut down on an angle to get the last bits out of the bucket.

It's literally my most used tool. I use it to mix, to lay out mortar for fixing stone, to skim coat the backs of tiles and veneer stone, levering stones up slightly to put more mortar in the bed, scratching my ass/back/legs, and I even occasionally cut myself on it accidentally!