They each have their issues; AC-u-KWIK doesn't have email addresses and it's pot luck whether the handler you pick 'of the 4 listed' actually has the best (if any) VIP facilities vs being a airline handler disinterested in pesky bizjets.
Handbook seems to be out of date, and other sources often incomplete for anywhere exotic.
We use a combination of the above, plus google, contacting more than one sometimes and seeing who is keen for the business and sounds half intelligent.