Legally and Constitutionally, there's no real basis any Ambassador becoming Acting President. The Constitutional line of succession ends with the VP with provisions for extending it by statute, and the statute ends with the most junior Cabinet Secretary. And there'd have to be not a single Congressman or Senator left, because quorum is based on the total number of active members: a single surviving Congressman could elect himself Speaker or a single surviving Senator could elect himself President Pro Tem, and either would succeed automatically to the Presidency. Without a Congress, they wouldn't be able to amend the statute. There would also have to by almost no State Governors left, because most states allow their Governors to make at least temporary appointments to fill Senate vacancies.
Practically, I'd expect de facto executive power to fall to the most senior surviving member of the military: in a crisis of this magnitude, commander-in-chief is the most important executive power, and the military chain of command provides a pretty solid mechanism for identifying the proper acting commander-in-chief. The Presidency would probably stand vacant until things were patched back together enough to hold special elections to refill the House and elect a new Speaker (who would then become President).
Supposing for the sake of argument that it has to be an Ambassador for some reason (a legal theory that the Ambassador in question automatically succeeds to Acting Secretary of State?), my first guess would be Ambassador to Russia, China, or the UN, those being the most important working ambassadorships. The US Ambassador to the UK is very prestigious, but for the last couple administrations at least it's been considered a sinecure to be handed out as a reward to valuable political supporters.