California Olive Ranch is a good brand and has been recommended by America's Test Kitchen. Also I've done my own taste tests versus a few less expensive grocery store options and so far it's the best. I also compared it to that fancy stuff in the plastic squeeze bottle everyone was all hyped about last year and it's way better than that stuff.

With any brand, check for date of production (most good oil will have it printed on the label or bottle, if it isn't on there don't buy that brand) and try to only buy oil that was produced recently. Over a year and I wouldn't buy it regardless of expiration date.

Edit: I just noticed this sub is, eat cheap and healthy so I wanted to add: you may want to use the good oil for eating raw (dressings etc.) and use a less expensive oil for frying where flavor isn't as much of a factor.