For a "plinker" and general fun...I'd go with the P22, however there have been some reports that the P22's are unreliable, prone to breaking, etc. I have NEVER seen one that didn't work as it should, but only passing along what I have read on various forums.
For a good "target" pistol...the P22 can't hold a candle to the 22/45 or the Buckmark. I almost picked up a Buckmark 5.5 Target model a month ago when another deal feel in my lap and I had to pass on the Buckmark. For me, the Buckmark and the Ruger are basically tied for first place, with the Ruger taking a slight advantage if you want to do any accessorizing/upgrading just by virtue of the Ruger aftermarket parts having a greater offering. Accuracy-wise...both are equal performers and with a little trigger tuning and a good shooter are capable of very competitive levels of accuracy!