Wow, i was just in the shower and was going to post what was the best movie ending since i just saw Planet of the Apes for the first time today.
As for worst... almost every Stephen King movie has a bad ending.
As for why i thought Planet of the Apes had a great ending... it was just a total surprise, and i love surprise ending (ie, Momento, The Game, Fight Club).
Ok, here's how i interpreted the ending: he left the planet, and General Thanes took over... the same guy that was saying how the graves would be unmarked so that when people go there, they would not be able to differentiate between the humans and apes, that they would be one people.
But why did he turn bad after all he said? I would say it was a fake front, maybe fear of Wahlberg because of the technology he had... or maybe he was honest and really believed it... but when Wahlberg left, and he realized he had absolute power (since martial law was instituted), it corrupted him.
Awesome ending though, i know lots of people said it had a bad ending, but i liked it.