Great topic. Personally, I find that EVOO is the x factor on a good pie. Whether you got a marinara with a dash of a pungent, grassy olive oil or a prosciutto and crimini pie with herbaceous, piney notes - it elevates the nose on the pie.
Olive oil is so underappreciated in terms of its aromatic qualities and diversity. There are more than 2,000 varietals of olive oil, nearly the same number of varietals as wine grapes have, and each has its flavor profile. So, I don't think it's a question of which olive oil as much as which olive oil for which pie.
It's worth going to a tasting bar or olive oil shop or better yet an olive oil tasting class to learn about the flavors of different ones. Basically there are three positive taste characteristics of an olive oil. Its fruitiness, its bitterness, and its pungency (that spiciness in the back of the throat). A good oil has a balance of the three and its peculiar aroma. An oil that is low in fruitiness, bitterness, and pungency is likely old or incorrectly produced (olive oil has a 12 to 24 month shelf-life depending on storage conditions - 60 degrees and unopened is ideal). Some of the ones that are relatively common and that I enjoy are:
Arbequina - mildly grassy or buttery, roasted nuts (depending on if its an early or late harvest)
Ascolano - sweet, stone fruit-y
Frantoio - Grassy, nettles
Koreneiki - grassy or banana-y (depending on if its an early or late harvest)
Leccino - pungent, green almond
Mission - artichoke, tomato leaf
Sevillano - Pine, eucalyptus
As far as purchasing olive oil in California, among the best producers for inexpensive but high quality olive oil are the super high density producers (mechanical harvest) such as California Olive Ranch (Arbequina/Ascolano/Koreneiki) and Corto Olive Oil (Arbequina/Ascolano/Koreneiki). They produce blends and single varietal oils, both of which are good depending on the application. In the grocery store they typically sell for around $15/liter, which is very reasonable. The artificially low prices of imported oils is caused by the EU giving subsidies to olive growers. Not to say there aren't excellent oils produced over seas and imported - but the vast majority of the Star, Bertolli, blah blah, is awful.
There has been a great deal of media attention on low quality imported olive oil from the Mediterranean, see the book Extra Virginity: The Sublime and Scandalous World of Extra Virgin Olive Oil by Tom Mueller.
The small-scale producers do charge a bit more, but compared to estate produced wines it is a true bargain. And, having worked for an olive producer for a couple years here in Solano County, Le Ferme Soleil - I can tell you it is an expensive and complicated food to make. They often use non-traditional varieties that are distinct and surprising: check out the results of the LA International Olive Oil Competition for info on some of the top-rated ones. Pacific Sun in Corning makes some brilliant EVOO. They have one of the best millers in the state in Pablo Voitzuk, and just make some great olive oil, not too expensive to order online from them. Other notable producers are Apollo Olive Oil, Yolo Press, Seka Hills, Bariani, and many others.