Best is a completely relative term. I take issue with people who say they know what's best, because what's best for one person might not even be enjoyable by another.
That said, my favorite oolong vendor, the one I'm most impressed with, has been Life in Teacup
. I like them because of (1) consistent high quality (2) reasonable prices (3) great selection of oolongs (4) some unique offerings I have found it tough to find elsewhere, and (5) wide range of grades and prices, from some relatively inexpensive ones up through competition winners.
Everything I've tried from them has been good. But...you're not me, so you may end up liking something completely different!