Anyone know the best way to unscrew the most unergonomic air cap ever?

Best bet is a cut down/champhered (check spelling) socket to fit perfectly I’d say. If you haven’t got one of those then a big, but a good quality adjustable spanner, with long arm for leverage that will cover the side of each face fully and not have too much play in the two separate heads of the spanner, which a cheap one might. Does that make sense.

You may well have tried that though

Vice grips

The dust cap type (usually blue) cap? I’ve found them difficult to unscrew if there is a slight leak from valve as it pressurises the void behind it. I found that pushing firmly down whilst unscrewing it helps.

If it’s the main air valve/nut thingy are you sure you’ve removed 100% of the air in there?