I've played a lot of systems and seen a lot of parts...hmm...let me start again.
It's hard to choose any one part of any one system since I run and play a lot of different games, each having some element or other that makes that particular RPG work well for what it specifically does.
I love the Stress/Panic Dice and its result from Free League's ALIEN RPG and have found it works really well for other settings where people are prone to freaking out and making bad situations worse (Red Dwarf and Ghostbusters for example).
I really love the base mechanic of Star Trek Adventures, where the more successes you roll the more information you can get or the more damage you can cause. Excess successes beyond what was needed to accomplish a task can be exchanged for additional or improved effects. This is a great concept and potentially very cinematic.
Two Japanese TRPGs I played years ago had awesome systems that I sadly can't recall in enough detail to duplicate them today: Mobile Racer Championship had a cool 'Shot Clock' mechanic for racing while Burning Spirit had a Martial Arts system similar to the Magic System in Ars Magica.
Thing is, I could go on and on, listing favorite parts from favorite systems extending my entire 45 years in the hobby. Bare in mind that for me, while its nice to have nifty parts of fun systems, rules in general are not my favorite part of the RPG experience. I think this will suffice for now as I have other, more interesting subjects to discuss.