I have tried cheap Kenda 1.5" slicks, Panaracer Paselas 1.25", and some cheap-o reverse tread 1.5" tires. The Panaracers were lightweight and felt fast, but they were too delicate for anything but paved roads. The Kendas performed well enough for thousands of city commuting miles (and even some singletrack!). But so far I am totally into my reverse tread 1.5" tires. They are basically a more durable version of my Kendas with some extra traction in muddy conditions. I would steer you away from the narrow 1.25" offerings, but otherwise you can't go wrong with 26" tires.
Peace
BK