Personally, I'd have to go for Beneath the Planet of the Apes (1970). The end, with the hero being riddled with bullets and then Charlton Heston setting off the doomsday bomb. Classic.
I voted for Escape from the Planet of the Apes, although the last time I saw the series I slightly favored the fourth installment. (Obviously, the original is best, but I think the sequels aren't half-bad for their budget.)
I went with Escape. I think it does the best job of capturing the social satire of the original. And Beneath loses points for the lack of Roddy McDowall and Jerry Goldsmith.
Escape and Conquest for me. Both run almost neck and neck on my list with the 1971 film set on 20th century Earth as the slightly better of the two. Battle has its moments but it's largely a mess and a very, very distant fifth among the original films.
Probably Escape, with Conquest a runner-up. It's annoying that with production values that were in the ballpark with the first film, Beneath The Planet Of The Apes had a script that was so flat and uninvolving. The best review of the film I've seen was this one: "Beneath The Planet Of The Apes is beneath the Planet Of The Apes."
For what it's worth, the novelization of Battle by David Gerrold is much more coherent than the movie, which appears to have been badly edited. The novel fleshes out the various plotlines more, possibly incorporating scenes from the original script. As it happened, I read the novelization before I saw the movie, and was disappointed that the movie wasn't more like the book!
I like Beneath. Lots of Nova. Those creepy mutants removing their "faces" to worship their god the bomb. The end (of everything) with the voice over “In one of the countless billions of galaxies in the universe, lies a medium-sized star, and one of its satellites, a green and insignificant planet, is now dead.” That part always gave me the chills.
Beneath had the very creepy and menacing pipe organ music to accompany the Bomb Cult's worship services and Victor Buono(Batman's King Tut) as one of the mutant leaders. Those aspects were effective.
Escape then beneath then conquest and finaly battle. Although the.Marky.mark Tim burron would come in last if it was choice. Rise may be 2nd if it was.a choice
Overall, I think Conquest is the greatest sequel. I really love it's dark and violent edge. Governor Brent is a great villain. That said, The last 20-30 minutes of Beneath is excellent. I like the end scenes as much as anything in the original film. I would rate this movie first if not for the fact that the rest of the film is mostly a retread and not that interesting.
It might be weird as fuck but I love Beneath. Unfortunately I've only seen bits and pieces of Conquest and Battle.
Escape... is by far the smartest and most enjoyable (I always loved that scene where the scientist explains time travel) but I like the apocalyptic tone of Beneath...
I don't remember liking that much about Beneath other than the abundant Linda Harrison screen time. And E-DUB, the one with Montalban is Conquest.