This is the kind of thing I could honestly sit down and ask questions about all day, fair warning. It could be a couple drinks. Really though, I find all this fascinating. What tend to be the criteria for selection? Is it dependent on the symbiont? These are all questions we can save too, if you'd rather.
Professionally? First Human S.P.E.C.T.R.E. Hands down, Spectres, which stands for Special Tactics and Reconnaissance, are the cream of the galaxy's crop, so to speak. The best of the best, multi-species, top tier military response team and galactic peacekeepers. Humanity had been struggling for a long time to get the kind of recognition in space as everyone else, and I was one of the first humans to ever be considered, and the first to be accepted into the program. It was a big moment for humanity and I like to think I've done my station proud with my conduct and service.
Personally? Giving a species back their home-world with minimal loss of life and bloodshed. Brokering a peace deal and an alliance between a warring A.I. faction and their creator, one of the hardest things I've done. Seeing one of my crew be able to breathe the air of their home-world for the first time in their lives? I wouldn't trade that moment for anything in the world.