10, 12, 29 al will do the job, perhaps the 12 is a tiny bit faster, but the 29 won't be anything faster then the 12.
The 5 watt will charge as well, be it slower and the charger will run hotter.
So basically any USB charger from any brand will do in a pinch, but the original apple chargers have a much more stable voltage with less peaks, drops and noise. They also seem to use better quality components and a much better design.