Took the wife to look at .22s this morning...

[Pathfinder]

I have the Sig P220 with the .22 conversion kit. Retail it's a pricey way to go (I didn't get either retail, BTW) but it's a great plinker, and the only difference between the two calibers is recoil. Lots of recoil in the .45ACP. OK, that and the cost of the ammo.

[Big Frank]

Of the .22s I have the 22/45 is my favorite, then the Ruger Mark II, and finally my NAA mini.

[tombogan03884]

I wouldn't own any semi auto .22 for plinking or competition except the BuckMark.
Training or carry of course have different requirements

[gunman42782]

I work at a gun range.  We rent guns, as well as sell them.  The Sig and Walther suck.  Very ammo sensitive.  I personally am not a huge fan of the 22/45 because I always hit the mag release when I am shooting it, but other than that, it is a fine, accurate, reliable gun.  The MK3s are more to my taste. (Actually think the MK2 is the best, and wish Ruger would have left well enough alone)  The MK series of guns lots of folks find difficult to put back together, but once you get the hang of it there is nothing to it. )  The SR22, although I find it ugly as sin, has proven to eat everything you feed it.  Someone mentioned the Browning Buchmark, another good choice.