I don't think there is a best strap lock. I've used several styles in the past, and they all have their pros and cons.
It's personal preference on which pros and cons you can live with. Here's my experience with them.
Dunlop StrapLok, dual design
Pros: works great, larger and longer screw for stability and strength, easy to install.
Cons: needs a little drop of lube a couple time a year to keep it operating smooth.
Shaller,
Pros: works great, easy to take on and off, easy to install.
Cons: the nut gets loose from time to time, and must be checked regularly.
Those rubber beer gaskets, and fender strap blocks
Pros: cheap, easy to use, and works well, Fender strap blocks work better than the beer gasket.
Cons: On some strap buttons with flat area the tight fit can loosen up the strap button screw.
Dunlop Ergo Lok Strap System
Pros: cheap, works well, no alteration to the guitar.
Cons: plastic, will wear out over time. And they only fit small strap buttons, not the aftermarket large buttons.
Planet Waves locking strap. A strap with locks on each end.
Pros: cheap, works well, they range in price and design, and no mods to the guitar, easy to use and stable.
Cons: They only fit small strap buttons, and the rubber back can mar the finish on the instrument.
So that's my take on them. Pros and cons.