My most used tools are: Nobex folding square. The smaller one is the only truly pocketable square I've ever used. This automatically makes it the most useful too.
Lufkin Multi Read measuring tapes. My preference is the smaller 6m x 25mm (20' x 99/100"), because its smaller form factor is more pocketable. It simply, easily & accurately measures both internal & external measures, without any gimmicky dust prone windows like the BMI measures do. It's cheap, too, & much more robust than even the best & most expensive laser (which are pretty good too).
English Stanley 60 1/2A block plane with holster. This short lived English "prototype" plane launched a legion of imitators some 40 years later from the likes of Veritas, Lie Nielsen et. al. A low blade angle for end grain, greater lever cap support of the cutting edge, extra long & wide sole lands for accurate shooting, narrow form factor for ergonomic ease and advanced (for the 70's) composite construction of cast iron, brass, steel & alloys & clever lateral thinking in design make for a "better mousetrap". Never popular, and marketed in Commonwealth countries only, this "unfashionable" tool consequently still sells S/H for almost giveaway prices, making it ideal as a daily user, more than capable of withstanding rough usage & not too upsetting to lose, break or damage from falls.