The absolute best finish Glock ever had, or ever will have, was the Austrian applied Tenifer finish. It had a nice sheen and color to it, good natural lubricity, was extremely hard, and it was virtually indestructible. Not to mention, you had to all but try to come up with a way to make a part rust that was treated with it.
Tenifer is somewhat similar to the Melonite process, but requires different chemicals and application procedures and temperatures. While many claim they are the same , they are not. Tenifer is superior in most every way.
The reason Glock changed the finish, was because the EPA has very stringent, costly requirements on the disposal of the chemicals used in Tenifer's application. Which makes it expensive to the point of becoming cost prohibitive to apply it here. It's not "banned" here as some have said. Just made to be very costly, because of government regulations regarding it's chemicals.
And it became cost prohibitive, along with creating logistical issues, for Glock to have the parts treated with Tenifer in Austria, (where the EPA has no authority, and where the process is very common), then shipped here for assembly into finished pistols at their New Smyrna, Georgia facility.
So Glock went to a type of Melonite finish similar to Springfield Armory's Melonite finish that they apply to their XD series of handguns. This is the finish that Glock uses today. It has a different color, (more of an ash grey), and has less sheen to it. While it can be deemed as satisfactory, many, including myself, believe it's not as durable, and more prone to rusting issues.
Simply put, Austrian manufactured, (Gen 1, 2, and 3), Tenifer treated Glocks have what many consider to be the best finish ever applied to a modern service pistol.