A trowel is the most used tool. A high quality trowel has a flexible blade created with a really high-grade steel that will withstand very hard usage. There are different patterns of trowels each created with different purpose 3 common types are:
Philadelphia Pattern - allows a lot of mortar, so it's is frequently used when working with block.
London Pattern - this is the standard trowel for brick layers.
Wide London Pattern - this is a multi-purpose trowel,  it has a rounded heel with allows more mortar to be forward, so it can be used by either a block or brick layer.
Keeping a trowel clean between jobs is really important, so they maintain great condition for each job.