I was reading through the other thread and finding it a little depressing - so many series with disappointing endings! - so wanted to see what examples people had of the opposite: book series with great, satisfying endings.
For me, two obvious ones are Return of the King (no explanation needed - has there ever been a better conclusion to a trilogy?) and Percy Jackson and the Last Olympian; whilst not my favourite of the series, the battle of New York is the epic finale the series deserved and the goodbyes at Camp Half Blood really feel like the end of an era. The sequels - Heroes of Olympus - never really clicked for me in the same way and had - guess what - another really disappointing ending...