i can only tell you about French wine... the French are pretty snooty about their wine and it's not easy to find non-French wine here in the supermarkets actually, just like many types of international foods, which is incredibly annoying, although i do live in the arse end of nowhere so i guess it's to be expected really...
all i know, when i'm in the supermarket, is i should only buy bottles that say they've been bottled at the chateau and not transported somewhere else and not blended by a cooperative - that's about it lol!!
luckily there's a really really good independent wine seller here who always recommends really nice stuff, for a reasonable price - you just tell her what you're making for dinner and she will find you the perfect bottle of wine...
we're really lucky in that we've got some lovely fairly local wines here too!
plus, the labelling here is still very much "region" based and not "grape" based - i have my preferred regions, but would be completely clueless as to what type of grape is involved lol!! i tend to like Bordeaux wines best i think, have never gone wrong with a nice St Emilion, and a good Chinon is nice too... and we drink Vouvray and Saumur sparkling wines by the GALLON LOL!!!
for sweet dessert wines, my all time fave is the honey coloured Muscat de Beaumes de Venise - little sips of heaven when served chilled on a hot summer's night