Do you just want vapour, or a balance with flavour?
Mutation X is a good value, and can produce great flavour when closed down, great vapour when opened up, or a balance. It works well with a Chuff Enuff wide bore top. CLT2 with Chuff top is great as well.
For maximum vapour regardless of flavour, I've had success with the Dark Horse with Delrin wide bore cap. The Doge Competition is good as well, with a Chuff top. With regular 510 style drip tip RDAs, try a Ninja-Mods Carb'd Black Delrin tip. Amazing.