We've all had 'em, things that really changed the guitar-playing experience for us. The idea here is to talk about true game-changers, gear that affected our playing to the point of changing our style or approach to musical self-expression. Here are mine:
1991 - PRS Custom. I'd been a Gibson SG Special player for 25 years by this time. Playing my first PRS was a complete game-changer. First guitar I ever had that could do pretty much anything. It played great, the humbuckers could be split for truly useful tones, and I rarely picked up the SG again.
1992 - Mesa Tremoverb. This amp was the main one in my work for 11 years, despite having several other amps in the studio at the time. No pedals, straight into the amp became my mantra. I've had amps that IMHO outdid it at its own game, such as the Two-Rock Onyx, but this one set a stylistic standard for me and changed my thinking in a way that was different from the way I used any previous amp.
2000 - PRS Singlecut. After finishing up a bunch of tracks for North American International Auto Show and nearly killing myself from lack of sleep in the process due to many last minute client picture changes, I treated myself to the SC as kind of a reward. For me, the guitar sounded even more vintage than the original McCarty, and I loved the pickups. This guitar taught me the glory of using a guitar's volume and tone controls, because it was so tremendously responsive. Because of it, my style changed some.
2002 - PRS McCarty Soapbar. I wasn't a fan of soapbar pickups even though they were what was in my old SG. This guitar changed my thinking, and for the next 6-7 years, I mostly recorded with PRS soapbars. And my playing changed to take advantage of the "hard edge" I got at the point of attack with the pickups. This forced me to start playing more precisely and to take advantage of this edge, begin using cleaner tones on the amps.
2012 - PRS HX/DA. I'd never been a Plexi or "M-style" player. Great amps, but not my cuppa meat, right? However, this amp has completely changed my approach, and is starting to turn the way I think about amplified guitar tones upside-down. It'll do everything I needed to do in the past, but it has a voice that really speaks to me in a different but very good way. I can see and hear my playing style move a little bit to take advantage of the amp's strengths.
As I said, this thread is about game-changing, getting you to do something different from what you're used to. I didn't mention my #1 PRS, which is my favorite PRS of all time, the Artist V; I didn't mention the Tonare Grand, which is absolutely the best acoustic I've ever played or owned. Or the CU22 SH Ltd that introduced me to the 57/08s. These things are great, interesting, and though I love them, they didn't change my approach to playing like the other things I mentioned did, they simply made what I was doing better (and that's a good thing, too!).