You'll get a hundred different opinions on that question! Most suppliers sell the same Chinese juice from Dekang. Some suppliers "tweak" these juices to their own specs. Personally, I only use the American made e-juice. Not because it's supposedly made in America but because I much prefer the flavor and performance. You'll just have to experiment to find what YOU like. My 2 cents is Eliquidplanets Essence line of juice. It's more expensive but worth every penny in my opinion. Halo also makes great American juice at a better price.
Try to buy a few sample sizes of juice from several suppliers.
PS. Nic level is misleading. I get a much bigger buzz off 16mg Essence juice than other suppliers 24mg juice.