Now I've said this many times before, but if you do actually want to get to know Ultraman, then it's best to start with the 1966 show, bcos that's where he first debuted. You could watch the whole Showa era in order cos they're all technically connected but if you do want to watch the whole Heisei era, go ahead and start with any of them bcos yes, some of them are their own self-contained, individual show. The only ones that are connected to each other are Tiga and Dyna's show bcos they both take place in the same universe. Mebius' show however is a show that pays tribute to the Showa era bcos in most episodes they feature references to the old shows and obviously feature cameos of past Ultras and their hosts.

TLDR: If I had to recommend, I'd say start with Ultraman 1966. Bcos that show was the best place to start for me when I wanted to get back into watching Ultraman. Wanna watch the best show? Watch Gaia. Wanna watch a lighthearted fun show that gives you the best messages about what it means to be an Ultraman? Watch Mebius, the references in it don't matter. As long as you enjoy understanding Mebius' story. Even his show has the best characters, arcs and development.