Queen(then Princess) Elizabeth of Great Britain was formally the Colonel-in-Chief of the Grenadier Guards, when she served as a Territorial Army driver. Obviously that's a ceremonial rank, and not a military one!

For militarily significant ranks, a quick scan of Wikipedia comes up with Lieutenant Colonel Haddon Donald in the New Zealand Army. I don't know if he's the highest-ranked, but it'll be hard to beat.

All surviving veterans are listed here so the problem might just be one of sifting through all those links.

King Michael I of Romania was made a Field Marshal in 1941. A ceremonial rank, obviously, although he did organize a successful coup in 1944 against the government of Ion Antonescu.

Interestingly, there are three living former heads of state from the WWII era, all of whom were deposed by Communist invasion: King Michael of Romania, King Simeon of Bulgaria (who was also the Bulgarian Prime Minister from 2001 until 2005), and the Dalai Lama.