If by best, you mean most successful, then in my case it is probably the drive system for the stop-start system which is becoming widely used on cars these days to save fuel when stopped in traffic. I hold the patents for a number of the drive coupling systems which allow these devices to work (unfortunately I don’t get all the money from them otherwise I would be a multi-millionaire!). This is something I worked on about 18 years ago, which is now being put into widespread use. That’s one of the most satisfying parts of being an engineer, when you see something that you designed actually being used.
I worked on a project where we designed and built a fully automated production line for an aerospace component. The project was successful and our client build at least 2 of these lines which are now in constant use at their facility. Solon these parts will be on many aircrafts!