The most logical one I've seen, and the one I personally use, is yy-mm-dd. It runs invariably from largest time unit to smallest, and is amenable to being decimalized, i.e. today is 2012.09.13.
Some wags refer to this convention as "stardates", but I merely find it convenient as it banishes ambiguity, and works well in logs
Unsurprisingly, my watch is on 24 hour, but not Zulu, time (I keep a Zulu time clock at the nav station). So last night I started building a fire at nineteen-thirty hours.
Your timing may vary.