If you had to choose a flag to be used forever, unchanged, how would it look like? Tonga made its choice already.
This is the flag of Tonga:
Since the 1840s, European missionaries arrived in the islands with the goal of converting the natives to Christianity. They were successful: according to last census, more than 90% of the population profess a branch of this religion.
When the first king of Tonga converted to Methodism, he adopted a flag consisting of a red cross on a white field, representing purity.
The flag was changed in 1866, putting the old flag as a canton of the red field, representing the Blood of Christ shed on crucifixion. A reason commonly attributed to the change was the fact it was identical to the White Cross' flag, adopted by the Geneva Conventions two years earlier. It's not clear if the similarity with a British red ensign was intended or not, since Tonga was then a British protectorate.
According the current Tonga constitution act, from 1875, this flag "can never be altered" and "shall always be the flag". Obviously, the flag could be changed after a new constitution. As this flag doesn't have political signs, the only reasons I can think to replacing it is either Christianism losing relevance on the country or people bore with this bold and elegant design. None of them seems near to happen, so it's a good bet if this flag lasts forever...
Comments are welcome!