It’s really quite interesting, lots of different top ten lists out there, MI6 usually top 3, number one is either CIA or another country which I would never have guessed in a century. No spoilers here!Look it up.
The Americans are certainly up there.
I suppose you can set criteria to judge by - and staffing and funding levels must be pretty critical, and history of internal security - double agents and defectors. Plus it’s more difficult to be an effective intelligence agency in an open, democratic society, where you have to at least pretend to play by the rules.
He most definitely should not be welcomed to the UK. 1. Because Sadiq Khan, and a host of other British Moslim politicians would not be welcomed in the USA. 2. The USA. is only Britain's closest ally when Britain follows them into useless unnecessary wars. 3. Because increased trade with the USA. means opening our doors to gene manipulated crap which Trump wants to dump somewhere.