Oh wow, I don't think it'd be possible for me to rank the entire series in order of personal preference!
Both because I've never been good at creating lists of favourites, but also because there are several films I've not seen in years and am due a re-watch, so I'd need to refresh my memory of several of them in order to really say how I feel about them now (I'm holding off on my re-watch of Godzilla vs Mechagodzilla until the anniversary celebrations this year, for example).
I first got into Godzilla back in the late 80's/early 90's when I was little, with the first films I saw on TV being Ebirah and I *think* Invasion of the Astro Monsters (it was plural in the UK, even though Ghidorah is the only "astro monster" in it
). After that I found our local video rental place had G'84 (and Gappa!), and by that point I was hooked. So those three have definitely always remained very high on my list of favourites and never really left.
The Showa era has definitely always been my favourite era overall though because the vast majority of Godzilla films that aired regularly on UK TV throughout the 90's were Showa films, so those are what I grew up with watching the most. As a result that's always been the era I've had the biggest nostalgic connection and love for, but even outside of that I've always just loved that "Showa look", even with other tokusatsu movies and shows. When I first got into the series it was still early on in the Heisei era, so that's always been my second favourite for that reason, even though I didn't end up seeing most of those films until I started importing DVDs in the early 2000s, since only G'84, GvsKG and GvM ever got a release in the UK.
The Millennium Series was so strange to watch for the first time, because after years of the Showa and to a lesser extent Heisei films being the entirety of what Godzilla was for me, suddenly I was watching brand new films that were way more modern in their filmmaking style than anything I'd seen before. They're not the only Godzilla films I don't have a big nostalgic connection to, but it's certainly the only *era* where that was the case (aside from everything that's come afterwards, obviously
).
I've been pleasantly surprised by how well so many of my childhood favourites have held up when I've gone back to rewatch them too. Ebirah and Son of Godzilla remain one of my favourite "duologies", and even though G'84 might not be up there with the very best, it's still a great movie that I think is a little underrated. IotAM is still excellent, though my opinion of Destroy All Monsters, which was a big favourite of mine growing up, may have lessened *somewhat* in later years. I still think it's really good, but it definitely lacks the engaging characters compared to films like Astro Monsters or Mothra vs Godzilla etc.
Destoroyah became a big favourite when I was finally able to see it, with Biollante coming right behind. King Ghidorah has remained one of my absolute favourites of the Heisei era though, and one of the few that got a UK release back in the day. GMK and Final Wars are my favourites of the Millennium era, though for different reasons.
I actually rewatched Megaguirus not that long ago, and I still feel like it's a little underrated, at least as far as not being the absolute abomination some people make it out to be. Don't get me wrong, it's never going to be on anyone's lists of favourites, but it's a decent middle of the road Godzilla film that I found to be better than the two Kiryu films that followed (though to be fair, those are two of the ones I really need to re-watch at some point).
Which is all one big, long-winded way of saying yeah, I don't think I'll be making a definitive list of favourites any time soon.