Most or at least many people would tell you Cafe Britt. And that would probably be my simple answer too -just "Cafe Britt". But, as many of my friends here will tell you, I rarely give simple answers to anything. So allow me to expand on that a bit.
Buying coffee in CR is not exactly like buying wine in France. Well it is in some respects but not others. Coffee in CR is most often sold by brandnames that have been developed by the major coffee mills. Those mills actually get their beans from a variety of farms in a region(s). It is not, or at least rarely, sold by vineyard, er I mean coffee finca. Oh you can definitely buy coffee at a coffee mill you visit and that mill will have a surrounding farm but there is no way to tell if the coffee you bought came from that field you just visited or another miles, sometimes of tens of miles, away. And the thing is coffee is in other ways like wine, if you're a true connoisseur, in that no one coffee or coffee growing area is considered absolute best from one year to the next. Add to that the fact the grapes, er I mean beans, get mixed in with others from neighboring farms to produce a homogenized product and the brand names become as much a product of marketing as anything else. Not that I mean to compare any CR coffee brand to Ernest & Julio Gallo.
The major EXPORT brands that most people seem to go for are Café Britt, Café Rey, Dota and maybe Naranjo. Of course there are others as well. Café Doka Estates also has a fine coffee which you can buy if you visit their farm but they market almost exclusively as a private label to Starbucks.
That last comment brings me to my next point - where is it best to buy the coffee? A lot of visitors buy coffee just the way many people buy wine in France or California, when visiting a winery, er I mean coffee processing plant. The 2 most popular places to do that near San Jose are Café Britt and Café Doka. There is something appealing about once you get home being able to serve some coffee to your guests and tell them that you purchased right at the farm (or to give such coffee away as gifts to your friends). Just keep in mind that the Coffee you buy there may really have come from someplace else fairly far away in CR. For example, Café Britt, which you can visit in the Poas/Barva Region (western central valley) also sells a Tres Rios (eastern central valley) product and a Tarrazu product (not sure of the region for this last but believe it is a little further south somewhere on the Cordillera de Talamanca.
Another place, I've heard recommended to buy coffee is by simply going into any of the larger supermarkets. This may be the least expensive place to buy it. To be honest, I'm not really sure where you get the best deal -here or right at the farm (promoted heavily at the end of your tour). Walking into a store can be bewildering. Picture as many different types and brands available as in a US store and then increase it. Just keep in mind that many of the brands are local products and not really of export quality (still good but not as great as Dota or Britt, etc.).
This last comment brings me to my final point - once you pick a brnad, what is the best type of coffee to buy? This is a tough question only you can answer as it depends in large part on individual taste. Do you prefer dark roast or light? Are you looking for something decaffeinated or espresso? Most real connoisseurs stay away from the products that have been adulterated with added flavors such as french vanilla, but I'm not that snobby (whatever you like is your business). Some like to mix different flavors in much the same way some people mix different types of orange juices h here in Florida to get the best balance between sweetness, acidity, pulpiness, etc. The thing about CR coffee in general is that it is already pretty balanced, some would say average or boring, but it produces just the type of cup of coffee most americans already like.
The last place, I might suggest to buy your coffee is where I usually do it, right at the Café Britt store at the airport on my way out of the country. I'm not sure how the price compares, probably not the absolute greatest deal. But you can't beat the convenience. No carrying around your coffee in your bags from one place to the next as you tour CR. You just stuff into your bags before you board the plane (or carry it in the special bag they're happy to sell you along with your coffee). And I also like to use up any leftover CR money that way, rather than having to carefully calculate how much US money I need to exchange on my last couple of days in CR.