If you are going to buy a complete rifle and the notion of swapping uppers on a real AR15 lower does not appeal to you, then the S&W M&P15-22 is the clear winner. If the accuracy is not enough for you, you can buy a match barrel for it for about $250 or so. The Mossberg is not getting a thumbs up from me.
Conversions that use a chamber adapter in an AR-15 barrel are, to me, a waste and cause too many problems.
When you go to dedciated uppers, there is starting to be a good variety, but some divergence. If accuracy less than 1 MOA is required, then you are going to get a Compass Lake for North of $1K. If 2MOA, maybe 1MOA but excellent reliability is more important, then the CMMG, Tactical Solutions, Nordics, etc are in the mix. The Nordics were one of the best, but if they come back in stock in the next 6 months, I will be surprised. If that is the one you want, you will have to order a JP22 upper. Nordic is swamped. Tactical Solutions is more .22 focused so their inventory is not lacking due to centerfire product demand like Nordic and CMMG. You can get a Velocity upper (.22 focused company as well) from Sportsman's Guide for not much coin and they are serviceable, but not the most accurate. There are a variety of others with slight tweaks here and there, but that kind of covers the landscape on the AR22 category.
I still think that most people will be better served with a custom 10/22 over an AR22 (or a Marlin 795/7000 if you don't like Ruger). I have all of them, and I shoot the AR22 with the Nordic upper on my match lower to practice for 3Gun only. When I plink, go for accuracy or play the speed games, I grab one of the custom 10/22s and for the precision match, the 7000. Accuracy and reliabilty with very good triggers can be had in the $400 to $800 range with a myriad of options. They are easy to work and on you don't need special tools.