Best Dark Chocolate?
Minimum 65% cocoa, and that is easily accessible and not too expensive. A good one is Trader Joes 3-pk Belgian Dark Chocolate (~$2.00 for 3 bars). I also like Dove Dark Chocolate squares.... I haven't really tried the fancy stuff yet.
That's such a subjective question. What constitutes "easily accessible," "not too expensive" and "best" are all highly idiosyncratic. For example, I work right next to a store that claims to have the best selection of chocolate bars in the Bay Area, and I live right across the street from a natural grocery that also carries quite a few different lines of premium (usually fair trade and/or organic) chocolates -- what's "accessible" to me is probably not what's "accessible" to most people! And since I regularly pay $6-8 a bar (and occasionally more), my idea of "expensive" is probably not the same as most people's either.
However, since you have access to a TJ's and mentioned $2 I'd suggest their Ocumare bars. TJ's in my area also carries Scharffen Berger. Both are in that same ~2 price range.
As for best, I tend to prefer South American/Caribbean chocolate to African chocolate -- I really like the smoothness, balance, and slightly floral qualities. I find single origin chocolate from Madagascar to be too acidic and with African chocolate that isn't single origin or chocolate of undisclosed origin, there's always the distressing issue of child slave labor in the Ivory Coast.
The best dark chocolate I've had recently was actually a 60 percent cocoa organic chocolate from the Grenada Chocolate Company (purchased online from chocosphere.com). My "standard" dark chocolate is the 65 percent with cocoa nibs from Vintage Plantations (usually runs me $3.39 at the store across the street).
Finally, you might also want to check out the growing number of high-cocoa solids (40 percent and higher) milk chocolates.
My whole extended family and most of our friends are dark chocolate freaks! The best that we have currently been able to buy......and use for cooking as well as eating... is California's Scharffen Berger. It's dear...but truly outstanding and the only chocolate I use in cooking. I have noticed recently that a lot of well known cooks call for it in their recipes.
"easily accessible and not too expensive"
Check out the offerings at your local Costco. Around the holidays they do especially well.