15-22. I love mine. You can get a new 15-22 for the price of, or cheaper than a dedicated .22lr upper. I have a Marlin 60, had a 795, and they are great guns, but the 15-22s are much simpler and fun.
I recently got a CZ 455 in .22 lr and then got a .22 WMR barrel. CZ also makes a semi-auto in 22 lr and 22 WMR. These are very accurate bolt guns. Another favorite is the Savage LV Mk II. I have a heavy barrel pre-accutrigger model and added a RifleBasix trigger. It is a very accurate gun but not as refined as the CZ. So there are lots of options.