best exterior varnish
Best ext. varnish is probably Epifanes into which you mix a little burnt sienna pigment for extra UV protection.
IMHO -- Best coating overall (gloss, longevity, adhesion, easy maintainence, etc.) is Honey Teak (Signature Finish) or 5 yr. Clear (Smith & Co.). Both are 2 part + 2 part acrylic/urethane copolymers that look similar to varnish (after sun fade). I use HT on a large application with no problems going into 4th year, can be buffed with high speed auto body shop buffer, lambswool bonnet and 1500-2000 grit compound if feeling lazy. Sanding is by 3M purple pad. Can lay-on several coats per day. Coating is best on a cool day. Very expensive. Use on teak only, not "softies" such as Iroko, etc.