I really should have kept my mouth shut to begin with in order to avoid the complications, but it's a great topic so I said something snappy. Really though the question is as fraught with nuance as "What's the best grape for wine?"
So much depends on how coffee is roasted, ground and brewed. Each style tends to select for different qualities in the coffee itself.
That doesn't change much even as you narrow the focus down. Different high-end espresso machines, using fines ground to the same size in the same grinder, handle particular coffees differently -- and that's the best high end machines, like top level La Marzoccos, Brasilias, Astorias, and Nuova Simonellis. And that's just multi-group, coffee house style espresso machines.
There's even more of a variation with mid levels, and still more variation when you come to different brewing systems such as drip (paper or gold filter, how hot the water, quality of water, etc.) and press.
Then there's the way the coffee is roasted. Not just light or dark, city or full city, but the roast profile it takes to get there, the cooling cycle, the size of the roaster, the efficiency of chaff removal, the type of the roaster (fluid bed, radiant, contact-drum) etc.
Luc, I'm relieved to hear you grind before brewing. Grinding after leaves the coffee lumpy and the grinder wet.
Food, Fair Trade is definitely a very good thing and it's right and ethical to buy from growers and traders who adhere to "Fair Trade" standards at minimum. You're right about Starbuck's, too. Starbuck's tends to roast at too high a temperature scorching the exterior but leaving the center underdone. Oddly this isn't true about Starbuck's in all cities. Apparently their roast masters have enough authority that those who know their craft can pull it off. Starbuck's copied its roast profile from Peet's. Peet himself, and Peet's (the chain) usually get it much better. The coffee has a unique dark caramel character. Very good for a French press.
I like Blue Mountain, but don't think it's the "best." The "best" depends.
FWIW, most people prefer blends rather than singles for espresso. Me too.
It's said that the quality of a cup of espresso comes from the four (Italian) Ms. In order (and Italian) they're, Miscela, mano, macchina dosatore, macchina espresso. In English, that's the blend (including how its roasted), the hand (skill) of the maker, the grinder, and the espresso machine.
At the end of the day, choosing the "best" coffee, exotic or not, comes from an understanding of the interdynamics of your brewing method and tastes.
In truth, my current favorite blend is made from specific estate Brazilian, Costa-Rican, and Sumatran arabica beans, with about 10% Indian robusta for zip and crema; batch roasted to City; then allowed to rest for between three to eight days before grinding; immediately dosed into a portafilter overfilled to 18 gms; up-tamped using the tamper screwed to doser/grinder to about 20 lbs (more of a Euro than an American tamp); and immediately brewed at 202F - 203F. But without at least a decent espresso machine (decent as in $1.000 and up); a more than decent grinder; and a good roast master, none of this makes sense.
Also, what I said about TJ's Safari Blend is true. It's a very good cup of espresso with wonderful chocolate notes, I can pick it up whenever I want it, ready for the grinder right out of the can, with a day or so of age out of the roaster already on it.
Anyway, it's complicated.
BDL