Conversion Kits - It's a tie!
There are two winners here because they're largely similar.
Go with a CMMG style bolt
There are a handful of other designs for conversion bolts, but none of them are as popular as the CMMG style. You'll have the most parts compatibility, support, etc. from these.
The CMMG conversion bolt is the gold standard from conversion bolts currently. They're ubiquitous and are available all over the internet. You're more likely to find one of these used because they've been selling so many of these for so long.
https://cmmg.com/22lr-ar-conversion-kit-bravo-no-magazine
One of the vendors who makes improvement accessories for the CMMG bolts also sells pre-upgraded bolts. On the site, they're only $10 more than CMMG but you get a lot of value for the money. They might be harder to find used, but if you're buying new, this is the way to go.
https://borebuddy.com/product/borebuddy-22lr-conversion-bolt-only/