The best project i worked on for selfish reasons was building my own house. I started the project with a small bit of family owned land, tracked down the owner of the access (who lived in another countrty) & negotiated access. Designed the house and took it through planning and dealt with all of the neighbours, found a main contractor, arranged for all of the sevices (that had to run a long way under a neighbours garden, sourced an alternative energy heating system and managed the installation all whilst doing the engineering day job. To see it grow out of the ground was a brilliant experience and i have ended up with a house that i could never have afforded to buy.
The best project I worked on was I think my dissertation at the university which involved analysis of the aerodynamic flow around the helicopter. I was working with world-class professor on this analysis. I was comparing the real flow from the wind tunnel around the helicopter to the aerodynamic flow around the same helicopter simulated on the computer. I created a program that compared the two and produced a difference.
This is amazing how the flow around the helicopter is getting disturbed while in-flight and the detailed analysis of it is fantastic.