Trangia burner

http://ultralight-hiking.com/stoves-trangia.html

Scroll down towards the bottom.

I've found that 1.75" is about perfect for using the burner without the simmer ring, wide open to boil water. If you are going to do a lot of frying then you might want to keep about 1/2" closer.

The absolute best burner design hands down.

See the following for ideas – http://www.youtube.com/watch?v=LHYLXp562-Y

of 1.25 to 1.75 inches is often described as ideal for most burners and most pots, whether using an alcohol stove or an Esbit burner (from top of Esbit tab). I favor the 1.75 inch distance in my own informal testing.