12 hour clock as I'm just so much more used to it. Anything past 12pm in the 24 hour clock makes me momentarily pause as I have to convert it.
I'm not in the military, so I don't need the clearness in communication the 24 hour clock gives (if radio signals are rough, the am/pm can get lost).
The 12 hour clock isn't just some British or American (how they love their imperial measurements) thing. Lots of countries use it.
Aesthetically, I think the 24 hour clock is ugly, and the 12 hour clock looks better than it even in digital form. Doubly so when spoken.
For those going 'there are 24 hours', a hours in a day are base 12. If you have a problem with that, go tell an ancient Egyptian.