Hard to name just one, as I've got lots of SAKs, Cold Steels, and Spydercos, more than a couple Opinels, Moras, and one Fallkniven S1. Guess if I had to narrow it down, I prefer my OH Soldier and Farmer for playing in the woods, SwissBianco Yeoman3 for in the city, and it tends to be a tie between a Spydie Delica4 and an Opi #8 for a single blade utility cutter, for all the mundane tasks.
Matt in Tx