The best that I've found, and I haven't yet tried many, are:
The Flavor Apprentice RY4 Double
The Flavor Apprentice Tobacco Blend (PG)
I don't get much of a caramel flavor from the RY4, but I still like it. It has a unique
tobacco flavor. I think it has their M-Type Premium in the blend, which is very hard to work with on it's own, due to it's extreme potency, but also good. Ry4 Double is great on it's own, but also mixes well with other flavors. I like to leave the RY4 Double as the dominant flavor.
Tobacco blend is a good mixer for a sharper flavor, as it has a lot of Ethyl Maltol in it, and so is pretty mild and sweet. It does seem to be a bit oily, though, as it's an actual tobacco extraction. I usually use it 50/50 or less with a strong artificial flavor.