Depends a little on what rim they are going in, but for general use I like Colby Valve stems.
(I won an emergency valve pack at an event last year and carry them around with me. Won't work for my HMMWV wheels, but if nothing else I can help someone else out.)
A little pricey, but cheap compared to calling for help and waiting for help to arrive and charge you.
www.colbyvalve.com
But those won't work for your exact situation, so you can ignore as necessary.
For a spare tire you probably want a shorter valve stem as you don't know where it's going to end up installed, and shorter has less chance of being damaged by outside forces when the spare is just bouncing around in the trailer, or something rubs up against it wherever it is mounted. The point of the spare is to get you to where the other tire can be repaired and put back into service, as when running duals you want them to be matched for wear so you aren't overloading the taller one. (probably not a big deal with most trailers unless you're running it commercially on a regular basis, but still something to consider)