I would have to say the time my contracting company got a gig to take over a t1 datacenter after a debacle and make it t2. Working on big, infrastructure level routers and switches, learning all the meeting-room-politics side of such a project, and learning the other intracacies of a datacenter that I "controlled" was extremely satisfying professionally and personally. On both the network engineer side and on the sysadmin side it felt the most raw and pure, close to the internet if that makes any sense.

Finally, I have to say some of the most rewarding personal experience has been working for extremely smart people. While working at a genetics lab I had the opportunity to learn from a pioneer in the sequencing field who got fed up with academia and was going capitalist. While working at an electrical motor company I got the oportunity to learn from one of the best devs I've ever known, a scala dude, and to learn from the what seemed to be the heir to the throne of the business and an extremely accomplished electrical engineer and scientist. I've also learned from and worked with some of the best warriors and leaders on the planet during my time in the Marine Corps.

I consider myself lucky.

A long time ago as a child I decided I knew not everyone was perfect, but you could learn to pickup a persons good habits and try to ignore their bad habits. Learning from the wisdom of other people is the most rewarding thing I can think of!