In order of preference: Safariland Comp II, Safariland Comp I, HKS, Speed Strip (Bianchi or Tuff Products), loose rounds, Maxfire.
The Safarilands are equally easy to load the gun with but the Comp II gets a slight edge for being easier to load the cartridges into (Comp I is made for models that the Comp II isn't though). HKS is somewhat slower and more cumbersome for me to use, but they are the most affordable next to speed strips and are offered for the widest array of models. Speed strips are convenient to carry and affordable, but no faster than loose cartridges. Maxfires are just awful: awkward to use, not compatible with most pouches, and liable to lose cartridges if carried loose in a pocket.
I've just bought a revolver that is cut for moonclips but I haven't used them enough to come to a decision about them yet. I've never tried Five-Star speedloaders either, but they would appear to work about like HKS albeit much more expensive. Finally, I know some people rather like the old Dade speedloaders, but those are long out of production and I've never even seen one in person.