What Bolt Stops/Releases are available, which is best?

bolt stops....

USGI only!
If anything was ever invented for nothing........THATS IT!

They are BIG. They are UGLY.
They do not fit the profile of the receiver to the point of making it look like it's grown a big wart or something.

AAAnd, they will make the rifle potentially unsafe. If you lay the rifle down on it's left side with it engaged, guess what? slam! The stupid things are so big, it releases the bolt. If you happen to have a loaded mag in the rifle, now it may be pointed somewhere you don't want a locked & loaded rifle pointed.

No way to get around that except to put a scope on the rifle so the receiver bolt on the scope mount will hold whatever you lay the rifle on away from the button. Or remember to always lay it down on the op rod side.

I would say that most people lay the rifle down on it's left side.

Use your money for something that will do something useful, like a NM op rod spring guide.

Besides that, well, there's nothing else really except, keep your rifle USGI.
You don't need any doo-dads, bling-blings, or potentially unsafe parts on it.