[Yamasa]

See if you can find a product called "Whip-It".
It is refined in S.Korea by the largest butane processor in the world.
It is also refined 9x. I put Ronson or Xigar in a lighter and I have to turn up the flame after using whip it, and vice versa.
It really is a noticeable improvement in burn, and it is usually cheaper that any of the others.

Also, as a side note, a product that falls right in between these 2 in terms of burn is Weller, which supplies butane for gas soldering devices. However it is not cheap.