While I can still remember my first adventure- AD&D in 1977 in the library of East Middle School in Aurora, CO (my Fighter was the last man standing vs a Purple worm...but I digress)- my true Golden Age was 1990-1994 in Austin, Tx, in a game group hosted by Alan Hench. (Hey, Alan!)
Even though I had played many systems by that point, and even though I have buddies I've been playing with in D/FW on & off with since 1985, THAT group was my crucible.
My skills as a roleplayer and GM really took off there. I ran my best campaign EVER there, and shortly after moving back to D/FW, ran my second best adventure ever.
That was due to 2 factors:
1) The players themselves. Up until recently, I've never gamed in a bigger group, and I got to see a wide variety of playstyles as a result. My current group is bigger, but the group is more homogeneous...not that there's anything wrong with that (

).
2) Most importantly, with them, I tried more game systems in that group- including a few playtests- than ever before. My personal collection went from a couple of dozen systems to over 100.
The reason was a rule
I suggested. We had had a couple of weeks where GMs just couldn't show up at the last minute, leaving us to eat our sandwiches and watch ST:NG and talk about gaming instead of gaming. I suggested that each week, we would have someone designated as the backup GM, so each week, we had 2 gaming possibilities. In addition, everyone in the group was responsible for running a campaign, so everyone could serve as a primary or backup GM.
In all honesty, some weren't up to the task, and were never primaries...but they knew it so didn't exactly cry about it.
Combined, the number of systems and the wide variety of gamers in the group pushed my game to improve on both sides of the shield.