So I just watched the original series and I was surprised how well it holds up. It's pretty much the quintessential globe-trotting adventure put to film. The first thing that catches attention is that it's not in any way a kids show, it could have easily been a live action serial as most stories are told in a kinda pulpy detective-adventure style, with everyone constantly getting punched, shot, exploded and strangled, wenches get kissed etc. What was really surprising is that there's no preset formula for the episodes - while you can always rely on Race to guerilla warfare through the natives, the show loves to constantly incapacitate and separate everyone, so pretty much anything can happen in an episode.
The animation is really nice, now, mind you, this is still Hanna-Barbera, so a lot of limited animation is in use and unintentional hilarity resulting from it, but together with that deep shaded comic book look and phenomenal OST it works, and still holds up better than a lot of subsequent cartoons (not to mention that shows today ape that style ironically), especially when you consider how ridiculously old the show is. It's so old, it predates even the original Star Trek, so imagine living in a time when there's pretty much nothing to watch or to do and suddenly this comes along. No wonder it became such a big hit.
Some choice episodes to check if you don't want to watch the entire show: The House of Seven Gargoyles, The Quetong Missile Mystery, Shadow of the Condor, The Devil's Tower.
Gonna be checking the 90s reboot now. Do stay away from the "New adventures" though, now that's trash.