NHT3
Grandmaster
I'll second the TX-22. I have 2, one I use for teaching and the other with an optic that I use for my own training.. As WT said, eats anything, easy takedown and a great trigger.. Several thousand rounds downrange between the pair and no issues and I find them very accurate.I'll throw a couple different handgun options that haven't been mentioned:
1) S&W Victory 22. Very similar to the Mark IV except I prefer it over the Rugers. I have 2 Mark IV's and 3 Victory's and I've had less issues with the Victory handguns. Plus it's super easy to switch out the barrel for a Volquartsen barrel, which is pretty sweet. They are generally $50 to $100 less than a comparable Mark IV.
2) Taurus TX-22. Compact sized striker fired polymer handgun that is relatively inexpensive and a great training 22. Mine has run fine with every type of ammo I've tried and it's very accurate. Not a bullseye pistol, but they have a competition version if you want that.