I'm sure it all depends on taste buds but I don't even know where to start!
You need to taste various oils and identify the ones that you like. If you have a shop that specializes in oils nearby, you can sample before you buy. Perhaps you can buy several small bottles and see how they work with various foods.
Be sure to use only oil from the most recent harvest--good oils should be clearly marked, and right now you want oil from 2013. Don't go by the 'use by' date--this tells you nothing about when the olives were harvested. The taste and health benefits of olive oil decline with age, so this is an important point.
For every day use, Costco's extra virgin olive oil is decent. Don't use an expensive oil for cooking, because you will lose the subtle flavor notes. Save those oils for dressing salads and vegetables.
This is one of the rare occasions when I will disagree with Maedl. I think Costco olive oil is terrible, but then I live in an olive producing region. For the record 73% of imported olive oils don't meet either IOC or USDA standards for "extra virgin." Also, an Italian (for example) olive oil can contain oils from practically any place. As long as it's bottled in Italy it can be labled "product of Italy."
I don't like Costco or Traitor Joe's olive oil because the flavor profile is what I would call "flat line". The most dependable oils are actually coming from California. Unfortunately the production for 2013 was relatively small because this was an alternate bearing year. This happens cyclically and is predictable. But then there was that pesky olive fly too.
For cooking, as in frying, I would use grapeseed oil because it has a high smoke point, meaning that you can heat it to a higher temperature.
Pierino, I usually don't think much of Costco's products, but I bought the bottle last month and thought it pretty decent for basic needs. It has an IGT EU rating, which I thought was a bit strange, plus it is dated from the 2013 harvest. I suspect that even as an IGT, it can contain olives from outside of Italy. I've noticed that even some of the expensive oils use a use by date and don't mention the year of harvest. California oil is good--but it is very expensive in my area, so I am likely to use oils from the other side of the Atlantic. Marketing olive oil is a dirty business, and 'buyer beware' applies. You are entirely right on the extra virgin issue--I take those words with a grain of salt.
I think we both agree that the best guide to olive oil is your own tastebuds. Leap at the chance to sample and to talk to people who understand how the oil is produced. that is the best education.
Re extra virgin olive oil... know your producer. And pay attention to the region (not just the country) of origin. That, along with the variety of olive, will have a lot to do with the differences in taste.
I agree with pierino about not cooking with the good ev's. Heat will destroy the nuances you bought them for! Save them for salads and drizzling. I'll admit to using "flat line" olive oil for cooking but never when I'm looking for taste.
Buy California olive oil, it is usually better quality and much fresher. I know it doesn't sound as cool, but trust me.
I am Italian and one might think I am biased on the subject, but Pierino gave you a good answer. Truth is, there are oils from Provence, Greece, Italy and California that are all gorgeous. Just make sure you can track the origin of the olives rather than the oil itself.
If you get a chance to taste oil, ask about what 'fruit', 'almond' or 'grass' undertones means. You will notice that some oils are more delicate, some sweeter, some piquant, some almost bitter. When you use it raw, the best is the one YOU like best.
This might be helpful: http://www.thekitchn.com...
I use the Trader Joe's Greek olive oil that's mentioned in this link and I'm quite pleased with it. Nice price, too! There's a link within that article, too, that you might want to read. Seems that a lot of extra-virgin olive oils aren't what they claim to be.