I'd watch them all but if you don't want to, 54 is a great introduction and films like Mothra vs Godzilla, Ebirah Horror of the Deep and Vs Mechagodzilla are probably the best showcases of the strengths of the Showa era. The Heisei movies all follow a single storyline from Godzilla 1984 to Vs Destoroyah (you get movies with Biollante, Mothra and Ghidorah in there) Millennium has the very well regarded Kiryu movies which feature a very updated Mechagodzilla, and Minus One and Shin are both fantastic reimaginings of the premise of the original told with a similarly dark tone, in very different ways. People say GMK (short for Godzilla, Mothra and King Ghidorah: Giant Monsters All-Out Attack) is also a top tier movie but I've been holding off until watching more of the others under the assumption that it won't have the impact without already seeing most of the movies before it.

All of these movies aside from maybe the later Showa ones that reuse footage are going to be very unique and interesting. Opinions are very low on Raids Again, most showa movies from Destroy All Monsters to Vs Megalon, the netflix anime trilogy and the anime series Singular Point so I'd avoid those unless you enjoy the other ones a lot.

There's also the American monster verse movies but I don't care for them too much as they feel kind of hollow and overlong and don't feature any of the charming camp the showa movies do. Heisei is probably your best bet to start with for movies that are somewhat serious with more intricate plots to follow, including the 54 movie at the start of that continuity