I've been looking for the "best" .22 for under $500. So far, I haven't found it, but have ended up with a Ruger MKIII Hunter, MKIII 22/45, MKII GC, Buck Mark Standard, S&W 22A, Walther P22 and Ruger Single Six. I've ended up with a case of RHADD (Rimfire Handgun Attention Deficit Disorder) because I can't decide which one to take shooting. My opinion on the ones I have.
S&W 22A: Reliable, well-built and easy to shoot accurately. Not much in the way of after-purchase upgrades. You can buy barrels and grips. Comes with integral optics rail. Mine shoots more consistently with standard velocity ammo. If you haven't looked at the 22A it is worth a look.
Ruger MK's and 22/45: Reliable, well-built and easy to shoot accurately. Target models come with optics rail. If you want to tinker and tweak, the Ruger after market seems to be unlimited. You can't modify the grips on the 22/45 without major surgery to the pistol.
Buck Mark: Reliable, well-built and easy to shoot accurately. Probably the least sensitive to ammo. Maybe the best out-of-the-box trigger. After market tweaks are limited. Most BM models come with one magazine and the optics rail is an additional purchase (add about $60 to get to what you buy with the 22A and Rugers.) If it matters, the optics rail replaces the rear sight, so you can't remove the optics and shoot with open sights.
Single-Six: If you like wheel guns, this on is a joy.
P22: It has the others beat in the light, small and "cute" category. Fun to shoot. However, the pistol is a notch below the others in design, materials of construction and (my) ability to shoot accurately. I've had to completely disassemble mine and do some serious parts polishing to prevent the trigger bar ears from eating the zinc slide. I still need to work on the extractor, because it does a fine job of throwing shell casings back toward my face or down my shirt.
With the Rugers, 22A and Buck Marks it's a matter of fit, feel and what you want to do with the pistol. I'd put them all in a "can't go wrong" box.