I have 'em all - the early standards in several variations, a couple of Mark I targets, several Mark IIs including the Government Target, one Mark III I hate, and one Mark IV blued target.
I loved the Mark IIs. Everything you need and nothing you don't.
The Mark IIIs were the answer to nothing - they just fouled up the disassembly/assembly routine unbelievably, had a magazine safety that prevented you from using it as a single loader if you had to, and the stupid loaded chamber indicator that was just one thing more that had the potential to foul up and go wrong. I think the gun was designed by lawyers.
In order of my preference:
Mark II.
Mark IV.
Ruger standard and Mark I target models.
The horrible Mark III.
Here are just a few of my Ruger .22s -
John
Ruger Standard (Red eagle 1st full year of production)
Ruger Mark I (bull barrel target model) - I shot this one in competition for years.
Ruger Mark II (Government Target)
THIS SPACE INTENTIONALLY LEFT BLANK (Mark III - BIG mistake)
Ruger Mark IV (Target model) - great takedown procedure.