Yet another vote for .25 Auto, with the .22 WMR coming in a close second. I've got a .25 "Raven" that my dad got for me as a birthday gift. It seems to be pretty dependable, but I have only fired 100 rounds with it so far. The first few shots are somewhat accurate, but after that the gun gets dirty fast and you can't hit a darn thing past 10 feet.
.25 is also under-powered. My plan, should I ever need to shoot anyone with my .25, is to empty the clip. One .25 has no stopping power, but six might, if I'm lucky. I have a friend who knew a guy that owned a jewelry shop. One day he was robbed and shot back at the BG with is .25, firing 5 times. 4 of the 5 hit the BG solidly in the chest, but he still was able to run out the door and walk 2 blocks before he died. So yes, .25 is deadly, if you can afford to wait a min or two...
Oh, I almost forgot to ask, I know CCI makes .25 ammo with speer gold dot JHPs. I'm thinking about buying a few boxes. Do any of you know if they are any good? My current ammo is winchester 50 gr FMJ. Should I stick with this or try out those JHPs?