I too asked this very same question a while back. To my surprise I got no answers. So I will attempt to help you with yours.
I also use 6v exclusively. I opted for the 510 style cartomizers myself. (I prefer the 510 Steel Mega 510 from Nhaler)
Using the Cool carts from V4L means you are running a cart with higher resistance. SO burning
juice should not be as big of a problem. It will however still burn some of them.
Flavors do not seem to be as big of an issue as where you get your
juice from.
If you are an espresso fan, the regular (not WOW) juice from V4L holds up very well.
I have yet to have any bad juice at 6v from Backwoods brew. My favorites are the Malty (like cotton candy), Honey Flue Cured, Maple Tobacco, Espresso & Melon Bacco. You do have to be careful with the Malty and the Honey Flue. Keep that carto really wet or they will taste burnt.
Backwoods Brew
I have also not found a bad one from Ms T Bakery. Hers all seem to hold up well at 6v. My favorites are her Espresso and her Chocolate Fudge Peanut Butter Brownie. Her juices pack a punch so be sure to order them in a lower nic level than you usually buy. I generally use 24mg nic, but with hers the 14mg is almost too much.
Chocolate Fudge Peanut Butter Brownies E-Juice - Ms T's Bakery Essentials
If you are into Peanut Butter (which I really love) get it from any supplier who sells Dekang Juice. This juice holds up better at high voltage than any other I have tried. I have not tried all the Dekang juices, but the peanut butter is fantastic.
E-Cig Liquid for Electronic Cigarette, Liquid for E Cigarettes | E Cigarette Liquid Selection including Flavored E Liquid!
I have also tried the Standard and Red Oak Samplers from Johnson Creek. To my surprise all of them hold up very well at 6v. I am not a fan of many of the flavors, but none tasted burnt.
Red Oak Smoke Juice - Johnson Creek
Another really good choice is Stricktly E-Juice. The prices are fantastic and more than cheap enough to buy a dozen or more small bottles to test and see what you like.
StrictlyEjuice.com, Flavor Shop!
Hopefully this will give you some places to start.