The current flag dates back to the middle ages, maybe as far as the 12th century. This makes it not only one of the youngest national flags of the world (since 2004), but also one of the oldest! (Even if it has been in disuse for most of its history.)

The red cross on white background comes from the flag of the late antique Kingdom of Iberia (located in present day Georgia and unrelated to the Iberian Peninsula). The four crosses are Bolnisi crosses, a Georgian national symbol, taken from an ornament found in the 5th century Bolnisi Sioni church.

Also check out their quite different naval ensign:

Wikipedia article about the flag

I found the flag on Wikipedia, but I cannot find any reliable sources verifying that it is actually used or officially adopted by somebody. So take it with a pinch of salt.

Anyway, there is an official flag of the Evenk Autonomous Okrug looking like this:

But I like the flag in the main image better, so that's why its the main image. And even if it might not be true, it is still a nice flag.

FOTW also lists this possible flag, that I'm not as impressed by:

Wikipedia article about Evenks

The provinces of Sweden are historical and cultural regions. Today, they no longer have any administrative function, but still serve as a basis of cultural identification.

The provinces do not have officially approved flags, but still flags have been created for some provinces and are in unofficial use.

The provinces do have coat of arms, and sometimes they are made into flags:

Wikipedia article about Provinces of Sweden

In my opinion a triangle generally looks better, but the trapezoid flags definitely gets points for distinctness, which shouldn't be underestimated.

The AK-47 and hoe of the Mozambican flag can also be seen as a version of the hammer and sickle, else the symbol is no longer used on national flags today.

The Angolan flag is derived from the flag of the ruling party MPLA, which led Angola to independence from Portugal:

Since MPLA won the Angolan Civil War, this flag has remained, although there have been plans to change the flag to something less politically loaded. This is a proposal:

If the winner of the civil war would have been UNITA or FNLA instead, maybe the Angolan flag would like one of their flags.

Flag of UNITA:

Flag of FNLA, which looks quite nice IMO:

Wikipedia article about the flag

Symbolism mainly taken from the coat of arms:

Some other redesigns:

From HansLN

From Reddit user u/imagiflaggi:

From Reddit user u/DerCriado:

From Reddit user u/Eunaotenhoesmola:

(Since you almost need a magnifying glass to see the small purple fields in the rainbows of the flags of El Salvador and Nicaragua, I think they barely count.)

The original flag from 1978 had the sisserou parrot looking the other direction, and a different color order in the cross:

The current flag dates from 1990 (apparently also adopted November 3).

Wikipedia article about the flag

(Or rather most complex, excluding flags with complex CoA.)

Wikipedia article about the flag

Wikipedia article about the flag

Here's an image with the flags and the prefecture names:

Wikipedia article about the flag

Wikipedia article about Alaska Day