What knot do you use?

[Nathan]

In general I let the style of the collar and the thickness of the tie material determine the type of knot.

This is absolutely correct. Different collar spreads and tie materials call for different knots. Like most have mentioned, the three I use most often are full Windsor, which I picked up in the military, the half-Windsor, which my father preferred and the four-in-hand which I find useful for more narrow collar spreads, or for thick ties that look fat and short when tied in a Windsor.