In my area, the 66-2 seems to be the most commonly encountered version with snubs running $600+ (4" guns are running about $500 these days and that is the most common barrel length). The -2 was the first M66 to lack the pinned barrel and recessed chambers. An older 66-1 or 66 no-dash have higher collector value, but the -2 is really just as good a shooter.
While it has the most collector value, I would not recommend a no-dash for a shooter. The changes made to the -1 were because of functioning problems with the no-dash rather than as cost cutting measures. The key difference betweent the no-dash and the -1 is that the gas ring was relocated from the yoke to the cylinder. This is because stainless steel expands a bit more when it heats up than carbon steel (the M13 and M19 never had a problem) and the gas ring would sometimes swell and lock up the gun when it heated up. Changing the location of the gas ring solved the issue.
Also, as is the case with all K-Frame Magnums, it's best to avoid magnum ammo with bullets lighter than 140gr as large amounts of lightweight magnums can eventually lead to a cracked forcing cone (the full-power 125gr seems to be the worst offender). A cracked forcing cone requires a new barrel to fix and S&W is no longer making K-Frame .357 Magnum barrels. If, however, you limit your use to either .38 Special ammo or .357 Magnum ammo with at least a 140gr bullet, your M66 (or any other K-Frame for that matter) will likely last several lifetimes. Truthfully, you're better off with the heavier bullets anyway as they will suffer less velocity loss from short barrels than the lighter faster loadings (158's usually run 1100-1150fps from a snub while 125's usually run 1200-1250fps).