Difficult question. But so far I love Nakamal at Home for their Stone, and I just ordered a batch of Wow from them. Their free shipping for me is also very fast, but I'm on the east coast, so I guess that makes sense.
GHK has some great kava also, although, for the same reason as above I suppose, it takes while for me to get a shipment from them. Last time was 11 days, as compared to 2 days from NAH.
BKH kava also has some very fast delivery for me, even though their kava for me, so far, has not been up to the quality of GHK or NAH, but it's always been good. I'm getting ready to order some Nambawan from them, and hoping this is as good as it's rumored to be.
Kava by Rex, I loved their Vanuatu3, very heady. The Tongan Lightening that I am currently just finishing off, is not as good, but it's acceptable quality. Their price can't be beat.
I think it's difficult at this time, as I'm still somewhat of a kava noob, to say who is really my favorite vendor. But the ones I listed above are all good and I will definitely order from all of them again.