Anti aircraft gun.....

which was best? From the smallest to biggest, all things considered...would the Bofors 40/mm be in the top 10? What about the German 128/mm zwilling?

20mm Flakveiling - took a ferocious toll of allied ac throughout the conflict.

I would suggest they you break them into categories such as
Automatic,
Heavy Fixed
Heavy
Naval,

Good idea Glider....let's keep those categories in mind...

Well I'm not an expert on guns... I've heard how good the German 88 was in combat, I guess it was pretty good AA too. I'm reading a book on Singapore, they only had the old 75 mm AA guns, the Japanese knew this so they flew over 10,000 feet to stay out of range. That is why the British introduced the new 94 mm (3.7 inch) which was very effective, but none were sent to Malaya. Also was an extremly good anti-tank gun, but with typical British efficiency, it wasn't available on an AFV until 1946! (as the 32 pounder on the Tortoise)

Well taking the easy ones first

Automatic
Has to be the 40mm Bofors, the only weapon I can think of that was produced and used by the USA, UK and Germany. Even the Japanese were trying to get it into production having captured some from the British.
I think that says it all.

Naval
US 5in L38, closely followed by the Japanese 3.9 Type 38 which was probably the better AA Gun, just however, the 5in had the proximity Fuze which had to give it the nod.

Fixed
German 128

Mobile, the toughest section of all
Not the 88 as it was to obviously a DP gun. This leaves the US 90mm and the British 3.7. My nod goes to the 90mm, both had proximity fuzes, both had good radar directors but the US 90mm had these earlier.