IMO, the "best bang for the buck" is Helmsman Spar Urethane. Pretty commonly found in hardware/big box stores. Use the Gloss- better UV protection than semi-gloss- and besides, you'll likely be rubbing it out in the end anyway. Rub the gloss out so it's dull, then use paste wax to bring back a pleasing luster.
If you want the very best, spring for a can of Epifanes spar varnish. Be prepared to be shocked when the man wants paid for it though. But, it has very high solids content and excellent UV protection. It is the darling of the brightwork finishers locally. (Brightwork is the exterior wooden trim on sail boats and power boats.) No greater test of varnish exists in the world. Again with the Gloss, if you will.
