Spanish Montana (the MTN 94 line more specifically) and Ironlak. These are paint created specially for aerosol art, so they have a really good drying, coverage and pressure. Both these cans are middle/low pressure, what I think it's better when dealing with stencils.
A New York Fat Cap it's the most recommended spray cap for this technique, as it can range from thin to thick lines. Also the coverage of this cap is quite uniform, the one displayed on the picture is a Ironlak New York Fat Cap.
Now this is really important: Your spray trace is determined by factors as type of cap, distance from the wall, pressure applied on the cap and speed of the spray.So here's a example that show the most common traces:
1- This usually happens when you put your spray can too close to the wall, please notice that it makes a solid trace without any blurring. This kind of trace is good when you're making a Graffiti piece trying to fill letters, but it's not that good for stencils, as If you cross the trace on the same point more than one time, you will probably make it drip and that's not cool.
2- This one is actually what I think it's the best for using stencils, not too solid and not too blurry. With a low pressure spray can you can do it without much hurry and worries about drips.
3- You should avoid this one, unless you're working with gradients as on a realist painting. This trace happens when you put your can far from the wall, even If you push the cap harder you will see that it takes more time to fuel the circular trace. Not recommended for stencils as they usually blur outside the safety border.
This is actually what I think is the best distance to get the second type of trace, don't forget to measure the strenght you put on the cap as it's one of the factor that will influence on making a stencil.
Now for some instruction on placing the stencil: you can't just grab it with your fingers and try to fill thru the holes, this way you will only get some really blurred shapes, even If you spray correctly.
You should stretch and push it against the wall, try to minimize at maximum the space between the stencil and the wall.