[KaijuUltraFan]

I started with the movie (Ultraman the Next), then watched the Zero Trilogy,..which I didn't wholly understand but I liked it and understood enough to catch what was going on (I just didn't recognize all the characters), then I started watching the original series and 80, and like Switchblade mentions Ultraman Max,..and I think Nexus (which tied into The Next movie). Mebius is another really good one.

Now I jump around depending on if I'm in the mood for Showa Ultraman or Heisei Ultraman.

I'd say the original series, or Max, or The Next movie is a good starting point.

It's really not a super complicated premise that's hard to understand. There's a lot of 'related' characters eventually that you come to understand their relations to each other, but by and large each series is going to be about an Alien that posses a human. That Human grows large and fights giant monsters as an Ultraman. That's the core of Ultraman.

Max or Mebius, (or both) probably would be a really good starting point though. Or the The Next (2004) movie.