Best brake booster

Are you looking for a replacement booster for an existing factory power brake system? Or are you adding power to a factory manual brake car? Is it a slant 6 or a V8?
If it's a replacement, there are no new ones. Your best bet would be to send it to Booster Dewy and have it rebuilt.
https://www.powerbrakebooster.com/
Don't forget to include the booster's separate vacuum tank when you send it in, it may also need attention.
If you're looking to add power brakes, a V8 has substantially less room than a slant 6. The 6 allows you to use many later boosters, but with the V8 you're pretty much limited to the '67-'72 7 1/2" and 8" Midland-Ross boosters with their offset bracketry. They're tight, but they fit. The trouble is that they are not available new or rebuilt, either. You'll have to find a suitable core unit and have it rebuilt.
Beware of some aftermarket kits that use generic "offshore" boosters (non-rebuildable and difficult to source replacements if/when one goes bad) that usually use some form of Ford or GM master cylinder, you'll have some issues with linkage and lines- nothing that can't be worked through, but additional work nonetheless.
Doctor Diff offers a booster and linkage that will work in your A body, patterned after the original parts, that accepts Mopar master cylinders. He also can set you up with the conversion lines and distribution block if you should decide to upgrade your single-pot master cylinder to a later dual-reservoir master cylinder (a VERY good idea); as well as being a one-stop shop for any disc brake upgrades you may have in mind (you also didn't say if you have drums or discs, or any intentions to upgrade). Good prices, great parts and stellar customer service.
DoctorDiff Drivetrain Components
Bendix Style Dual Diaphragm Booster for Mopars
And speaking of master cylinders, people will tell you to skip the power and just install a 15/16" bore master in it- which does work very well. But I also understand the wife wanting power brakes... Catch 22.