Who holds the world record for reaching the landmark of fifty runs off the fewest number of deliveries in top-flight international cricket (i.e. recognised and official matches in terms of the game's govetning bodies)?

International cricket (and not domestic).

In 50 over match it is held by Jayasuria (Sri Lanka). He did it in 17 balls. I remember it as he did it against us. He broke Simon O Donnel record of 18 balls. Since then Afridi twice scored 50 in 18 balls (missed by 1 ball).

In a 20 over match, the new record was just made by Yuvraj (India) in just 12 balls. It included 6 sixes in an over.

Yep. Perfect answer. And all the best to Inzamam, by the way - I'll miss the guy.