Best? Depends on your taste.
Do you mean FAVORITE?
Depends on your goal. Fraps have a ton of sugar and calories but some don't like hard core coffee drinks like a latte. The beauty of Starbuck's is that they'll frequently let you taste a drink without buying it if there's not a huge rush going on. I suspect you are curious about a drink or two. :-)
Do this... next time you hit a Starbuck's, just ask the barista if they like the drink you're most curious about trying. Frequently the best barista's are dying to whip something up and get you to try it. They'll even have custom tweaks they can show you! Best time to do this is when its slow. If the barista sucks just go across the street to a different location! ROFL...
Suggestion: Chai Latte for taste, but ask for a Doppio if you are going for effect. :-) DRINK IT!!