Favorite Aerospace System

What is your favorite aerospace system (i.e. engines, planes, rockets, or anything else) and why? Figured I'd ask because I'm a little bored of studying and I am interested to see what you guys think.

My favorite overall would have to be the SR-71 based on its flight history, but mainly its design. I've been able to see one in person twice and both times I couldn't help but get excited to walk around it. I also find the J-58 that it uses to be fascinating. I would have loved to be in the meeting where someone threw out the idea of making a hybrid turbojet/ramjet.

Runners up would be the X-15 and the XB-70