The best speed-loaders are somewhat bulky, to carry concealed, so, arguably, one’s best first-up speed-loader, in a street-carry context, is a second revolver. More than two revolvers can start to get a bit heavy, of course.
I was mandated to use Dade loaders, during my police academy training, in 1983-1984. These were very fast, and the simplicity of design was reassuring, but if dropped, the weak spring would not retain the cartridges. The “theory” was that if the rounds separated from the loader, upon hitting the pavement, the individual cartridges would not roll far, whereas most other loaders would bounce and roll away, carrying all of the cartridges with them. I did not buy into that line of thinking, so, will say that Dade loaders are not a best choice.
Upon swearing the oath of office, at graduation, I was free to choose, and liked HKS. I still do. No failures. The clock-wise direction of the twist, to release the cartridges, however, may be more intuitive for operating the loader with the right hand. Those right-handed folks, who want to hold the revolver in the right hand, and reload with the left hand, might well find the push-to-release loaders, such as Safariland and SL Variant, a bit smoother. (I am left-handed, and functionally ambidextrous with revolvers, and speed-loaders, but can certainly understand how a right-hander would find HKS a bit challenging to operate in the left hand.)
When the Safariland Split Six belt carrier became available, I was carrying an S&W Model 58. At the time, one could not get a Split Six for N-Frame loaders larger than the Safariland Comp I, so, I bought a couple of those, in the S&W .41 Magnum size. Other than having to remember to push, rather than twist, the Comp I worked fine, for me, 1985-1990. (I then switched to using a Colt Commander for duty and personal-time carry.)
When I returned to using revolvers for street patrol, in 1993, after experiencing problems with two auto-loaders, I thoroughly tested Safariland Comp II, and have nothing to complain about, but kept using HKS. (In 1997, I resumed using auto-loading duty pistols, but have continued to use revolving pistols for back-up, and for personal-time carry.)
I only recently obtained pre-owned samples of the SL Variant. They seem quite well-made, but I have yet to really put them through through serious testing,
What is the “best” speed-loader? Well, except for the Dade, I am not sure that anything I have used is not plenty good enough.