Hello KayKay,
There are so many vendors that offer quality juice, there is no way to single any one vendor as the best. Taste is so subjective, the best way to discover an eliquid that you like is to order sample sizes from various vendors and see what tastes good to you. Many vendors offer individual sample size bottles or sample packs which gives you the opportunity to try out several of their flavors.
I do buy Tasty Vapor doublers and mix with my own nic or vape them just as they come with no nic. The smallest size doubler you can purchase is 2 oz. so you might want to start with smaller bottles until you discover the flavors you prefer. As Don pointed out, the doublers are not double flavor, just regular flavor with 0 nic.
If you have an interest in a particular vendors juice, read the reviews on their forums here on ECF. That's usually a pretty good indication of quality and taste.
Beckah