ISO must get everyone's vote, since it is a direct combination of ASA and DIN e.g. ISO 400/27 is ASA 400 and also DIN 27. DIN has a small further benefit in that you can usually find DIN in comparison tables for the really old speed systems like Hurter & Driffield (H&D). I force myself to say ISO and not ASA (and also use metric units) in deference to the younger generation who have not been taught imperial units, but use common sense when referring to "metricated" products in the UK which of course have not been metricated at all - 4x5" film is an inch size, it always will be, no one in their right mind would make 10.2 x 12.7 cm a standard size when there is the perfectly good "real" metric size of 9 x 12 cm. And then there are the anomalies that you just learn to live with, like tire sizes - 165/13 means a tire with a tread width of 165 mm to fit a 13" wheel. And engine sizes - even before metrication we in the UK always specified engine capacity in cc, never cubic inches. And there's engineering hardware - down at the DIY supermarket screws are labelled for example 6 mm/1/4", which causes me to scream with pain - even if 1/4" were exactly 6 mm, it would still have a non-metric thread pitch. But what the hell!!!!!!!!!!!!!

PS: German "Fahrzeug" means "vehicle" - an aeroplane is a "Flugzeug". "Ich habe Flugzeuge im Bauch" means "I've got butterflies in my stomach."