Pavel's 3-5 was pretty instrumental in getting me to quit messing around and focus on what matters. It also taught me valuable lessons in deloading, which I quickly forgot for about 5 years and then wondered why I couldn't progress like I used to. Also taught me how certain movements carry over into others.
20 rep squats was huge in teaching me that psychology plays as big a role as physiology in terms of progress and success. It also taught me that I wasn't eating as much as I thought I was, how to rest pause effectively, and that you can go almost completely insane when you only hit 19 reps instead of 20.
DoggCrapp broke me out of a 3 year slump where I was just going through the motions by radically changing everything about how I trained. It also got me to start deadlifting again after a 3 year hiatus due to an injury by introducing me to partial deadlifts without feeling like I was "cheating" or "being a wimp" by not pulling from the floor. Tons of great intensity modifiers as well which are great to throw into other programs. It was also probably some of the most fun I ever had training, and crazy considering you're only really doing 1 set per bodypart.
5/3/1 finally helped me understand how assistance work works: as long as the main lift goes up, your assistance work is working. Prior to this, I used to stress out over assistance work, carefully monitoring ITS progress vs the main lift and wondering how to improve my assistance lifts. BBB also had the most radical transformation on my physique, allowing me to gain 12lbs in 6 months without meaning to.
I've posted this part a bunch, but my wife's co-worker met me before I started the program, and then I hadn't seen him until I gained all the weight. He saw my wife and I walking from a distance, and the change was so substantial that he thought my wife had divorced me and started dating a completely different person in the span of time since he had met me. He completely didn't recognize me.
Finally, my current program of no program has been my most successful so far. Stolen a bunch of different principles that have worked from other programs, using a lot of ROM progression, assistance work as needed, intensity modifiers, etc etc. Making huge strides while learning a new sport.