Man. I'm geeking out right now that you're enjoying Godzilla, seriously.
The original film does have some slow points and I think that's one of the biggest criticisms I've heard about it. Which, when you take that for face value, that actually kind of speaks for how well the film really is done, doesn't it? You're like the fifth person in my life who I've told to see the film and say "No, it's really good. Trust me." Only to have you come back and say "Oh wow. You're right. That was pretty decent."
I have so much to say. Where to begin? If you were actually able to watch Godzilla Raids Again and Godzilla x King Kong, you'll actually be able to watch almost any Godzilla film and enjoy it. While I personally think watching them all in order is a bit of a trek, (though I literally over the course of two days watched all of the Heisei films on Hulu in order), if you do do that, you'll get to see how the character has evolved over the years.
For the Showa Era, as the franchise goes on, Godzilla will become less of a monster to be feared and starts becoming a protector of Japan/Earth. The suits change to make him look friendlier and the films will tackle other issues such as pollution, bullying, what have you. I promise you that color balancing gets better much quicker as the films go on, and the special effects get cooler and weirder and more creative. You'll also pick up on things here and there, like for instance, to save money Toho would recycle shots for films, so they'll re-use footage for two different films like Godzilla X Gigan and Godzilla X Megalon and you can see how sometimes when they bring suits out of storage for a second film there's notable wear and tear from a previous production. One of the things I'd recommend, after (or even during) you finish a film, look up the goofs and trivia for it on websites like IMDB. You'll find some pretty cool stuff. You'll also find that as time goes on, Godzilla gains and loses new powers all the time. Flight. Psychic abilities, what have you.
A lot of people really like the Heisei era, for it's continuity, but it's probably my least favorite era. Like I said before, the monsters look FANTASTIC all throughout that era, but a lot of them are very limited in motion and to make up for that, you're gonna get beam battles galore. Everything might look better than the Showa era, but some of the energy and enthusiasm seems to be missing.
The Millennium Era is probably my favorite. Favorite suits, favorite stories, just fun all around. I also think it strikes the right balance between special effects and monsters just going at it. Just trust me when I say, Godzilla X Megagurius is a bit of a mess and Godzilla: Final Wars is only fun if you're willing to turn off your brain.
As an aside, one of the best things about watching all these films, is because they're all pretty dated now, you get to see trends in technology and fashion and cars and such evolve right before your eyes. It's a little something extra that kind of makes shit fun.
Dude. Yeah, keep watching and if you wanna talk Godzilla shout me out and I'll totally join in on you. If I were to offer two bits of advice, it'd be first maybe don't try to watch the entire franchise in order, for fear of you getting bored or tired of it. Jump around. Most of the stories are self contained and continuity doesn't matter that much for Godzilla. Second, and more importantly, there are two American Godzilla films. There's the Matthew Broderick one from the '90s which is Godzilla in name only. It's basically a whole movie about a giant lizard playing hide in seek in New York City from a military that couldn't find its own butt if they were drawn a map to it. If it was named anything else, it'd be a decent Kaiju film, but for something carrying the name of Godzilla, it's disappointing. The 2014 film though? IT'S KICK ASS! Watch it on the biggest screen with the best sound you can get. Sadly, Godzilla actually only shows up a bit here and there in the movie, but when he's on screen, SHIT IS EPIC! Watch it once, then wait a few months, and watch it a second time. For some reason, the movie is better on the second watch.
Godzilla is awesome.