I have to agree with you, the Internet is probably the single biggest advance we have all seen, both for modeling and in general. IMHO, the advent of resin and photoetch Am parts is #2, and the great expansion of availability in obscure aircarft kits would round out the 3.
Definitely internet has to be #1 in all facets of society: hobby work and general usage.
for the hobby specifically:
I’ll go along with the comment about improved acrylic paints, I don’t dislike enamels, but you really do have to be so much more careful with them and the cleanup is more of a nuisance than with acrylics.
The rise of the limited run and cottage industry end of things, were it not for them we would have no chance to see a lot of the more obscure subjects that the mainstream won’t touch. The fact that a lot of limited run kits are coming close, if not equalling (and occaisionally exceeding) mainstream kit in quality has a lot to do with why I think they are a great inovation.
I’ll say resin is a great inovation for making conversions. Photo etch can be handy at times, but too much a pain in the rear sometimes for me to call it a GREAT inovation.
The internet would be number one. For number two I would have to go with the introduction of improved acrylic paints. Number three would definately be resin AM products.
Now, here is a thought for you. What do you consider the most important innovation used by modelers? For me it would be the airbrush.
I have to agree with the airbrush for modelers - this tool was available for years, but the first generation ABs were hard to use and required much practice. The double action brush has made it possible for the average user to do credible work. CA adhesives is surely a close second!