[Fish Sandwich]

My advice for recommendations is usually "whatever sounds interesting to you probably will be", but honestly, in this particular case, I think Ultraman Orb is what I would most recommend as a gateway series. It's readily available to watch on Crunchyroll, there's no continuity whatsoever to worry about going in, and it does a great job blending the traditional Ultra format of anthology style one-off episodes with some more character-driven, long-term storytelling. Between all that, a tone that balances comedy and drama evenly, and the usual stellar production values, it's a very well-rounded, complete package of a show that I think would be easy to get into.

Definitely don't be afraid to go for anything that catches your eye, though. There's a really varied library of Ultraman shows out there, and I don't think any of them make bad places to start. For example, Geed and Mebius both lean heavily on decades of continuity, but both present themselves in a way where knowledge of all that isn't necessary to enjoy the show. Nexus could be considered a poor starting point for how its style and tone don't reflect the majority of the franchise, but if a darker, heavily serialized show sounds like more fun to you, it can't hurt to give it a shot.

Basically, I say don't worry too much about finding something to "represent the series overall" or anything like that. Just find something that clicks with you and go from there.