The best wine at WDW is on the "reserve" list at the California Grill. The regular list is pretty darn good, with most offerings available by the glass or bottle. The reserve list is bottles only. While it changes as inventory depletes, you will usually find Pinot producers such as Martinelli (single vineyard bottlings), Etude, Williams-Selyem, Kistler, Loring, and Merry Edwards. The last time I was there I was able to get a Martinelli "Bella Vigna" by the glass. While this is Martinelli's lowest level Pinot, it is still a terrific wine and it was great to get it by the glass as my dining partners did not drink wine which moved me away from the bottle selections. The CG is one of the restaurants "detachced" from the rest of the corporate wine buying at WDW, meaning that most every other restaurant has to select from the inventory that corporate Disney buys. The CG is one of the few that has authority to go out in the market and buy its own wine. So most of the wines that you will find there cannot be found anywhere else on property. You do not need a reservation to go up to the CG for drinks. If you just want a glass of wine, the hostess(es) on the second floor of the CR will let you up.
The next best wine is at V&A, but to partake of their wine, you need a reservation and a committment to spend a lot of money for dinner.
Epcot wine is actually quite average. The restaurants in the various countries tend to pour "supermarket quality" wines both by the glass and by the bottle. Even Bistro de Paris (which is the highest level restaurant in Epcot) has a very pedestrian selection of Burgundy. Not bad, mind you. But certainly nothing memorable.
BlueZoo in the Dolphin has a very nice (though a bit pricey) wine list.
If you are feeling adventurous and want to expand your horizons, go to Jiko at Animal Kingdom Lodge. It is another of the "detached" lists and they buy their own wine. The list is entirely South African and some of the better producers from that country are represented quite well such as Warwick, Ernie Els and Rustenberg. While South Africa has some decent Pinot, it is more known for its Pinotage, which is a hybrid rooted from Pinot Noir and Cinsaut. South Africa is the only country to produce this varietal seriously and you might want to check it out. South Africa also excels in the production of Cabernet, Syrah and various blends that incorporate all of the aforementioned varietals.
Let me know if you need more tips. I could go on and on.