In your opinion what were some of the best benefits of having diplomatic immunity?
Somewhat more seriously, pretty much the only perk was getting to use the Diplomats & Crew line at airport immigration. Most countries, including mine, make their diplomats pay their parking fines etc, and while I was theoretically exempted from security checks, it was virtually always faster to put your bags through the X-ray and walk through the metal detector than it was to pull rank and wait forever as the proto-TSA at the time ran around like headless chickens, looking for the manager's manager's manager needed to authorize the exception. The police treat diplomats with either thinly disguised contempt (if they ran into them a lot and knew they couldn't do much of anything) or, more commonly, puzzlement; and once again, it was usually way easier to pop the trunk and let them have a look than it is to invoke immunity and deal with the resulting confusion.
And as far as complimentary upgrades to first class and "ambassador's lounges" (wot?) do, I'm not sure what the other reply has been smoking, but it's probably not the Vienna Convention. Airlines don't give any free benefits to diplomats, they earn their status through butt-in-seat miles like the rest of us.