What is the best preservative and method of application for a wooden vertical board fence?

The fence was installed almost two years ago and is certainly ready for a coat of some kind of preservative, but I really don't know what type/brand is best.

Also, what about application? I'd be glad to spend the dough on whatever kind of gear makes the job easiest and most convenient, without getting into ridiculous amounts. Regular compressor/tank/spray gun for high-pressure/low-volume application plus possible use of other air tools (air ratchet, impact wrench, etc.); specialized high-volume/low-pressure paint application gear which could be used for other painting applications; power roller; Wagner "Power-Painter"-type gear; hand-applied with painting mitts?