I should like to preface this by saying it's solely the original series. I've seen the Burton film once and two of the new-ones ages ago once but it's the originals here.
I recently got the original boxset on the cheap and figured, of course, run through them. It's an interesting series in its way and like most of its kind, you sort of wish they stayed to the first one (like I once watched Return of the Seven and for a while it tainted my view of The Magnificent Seven).
Planet is by itself a classic. Now I by now know what's coming, it's the most watched of the series but there's still that element throughout that maybe Taylor isn't on Earth all the same. The film makers doing a fantastic job early on of finding locations that must have lain the seeds in audiences mind in '68 of, well, maybe this *is* an alien world they're filming on. Goldsmith's score is evocative throughout and more-so early on. Heston is on form and the supporting acts also. It's so removed from the book of course (the Burton film has its detractors but the ending is at least quite true to the book in a way) but there's something haunting about the film. Maybe it's the fact its set far into the future and anything we knew has turned to dust.
Beneath has interesting ideas but I found on this latest viewing I sort of lost interest along the way. And it ended in a dark, haunting way and that's how it should've stayed. However...
Escape is the best of the sequels. The idea that these guys somehow escaped 3955 by the skin of the teeth is plausible even if the impression you get in the first two films that for all their evolved intelligence, the apes haven't mastered technology as such never mind send a spacecraft up. The cast isn't too bad, Braden gets more unhinged as it goes on and William Windom is great whatever he does and then there's Montalban at the death. The end again feels quite haunting. Just the long shot and everything.
Conquest again there's an interesting idea, if anything the look of the film helps. The exterior LA locations feel quite futuristic in its way (one reason you suspect Buck Rogers in 1980-81 used some of them) and interior shots you feel it's 1991 (or their idea of it) irrespective of it in other ways looking 1970sish.
Battle the most pointless one, it has the look and feel of a cheap TV movie. Conquest did enough to lay the seeds for Planet and Battle should never have happened.