Banner of the Stars III OVA is easily the best of the entire series. It comes full circle back to the planet Martine, where Jinto was born. He is returning to govern the planet but Martine wants independance, not to mention that half of their population hates the Abh. Banner III is about this conflict, as well as Jinto's relationship with Lafiel who has taken a leave of absence to be with him.
I cannot reveal too much more about the series without giving spoilers but the character development in this 2 episode OVA is outstanding. Every character has grown up and changed, even Diaho the cat. The plot is tight and the pacing is just right.
The ending is outstanding. Many loose ends are tied up but there is always room for further sequels in this universe. This is how a good franchise should end, if it is indeed the end. With people wanting more, but not really needing more because it is a "satisfying" ending.
If future ones are as good as Banner of the Stars III then I hope they make a 4th, 5th and 6th series.