If i were to watch only 1 godzilla movie... which should I watch?
Depends what you want.
If you want the most interesting movie, watch the original 1954 one. The Japanese language version if possible. If you want to count it as the same movie, watch the english language re-cut with dubbing and the inserted Raymond Burr scenes too. But if you only want to pick one, I'd do the Japanese version with subs. Neither is the corny saturday afternoon matinee fare that most people associate Godzilla with. The Japanese cut is very grim and sombre and takes its theme of Godzilla as an A-Bomb allegory very seriously. The US version takes out all the anti-nuclear message (understandably given the climate of the times) and is a little bit more straightforward as a "monster movie" but it still takes itself seriously and doesn't have much cheese. And it's interesting to see that they did a very good job overall of making it seem like Burr is integral to a movie that had nothing to do with him or a character like him when it was shot.
If you're looking for something that's a bit cornier and more just the fun of people in rubber monster suits slapping the **** out of each other... Oh god. I'm not sure if I can pick just one.
If you're cool with more black & white, Mothra vs Godzilla is pretty solid. A lot of people will tell you it's the best one after the original. I'm not one of them, but that's just my own tastes and maybe I'm weird like that.
If you want something a little bit more modern, Godzilla vs Mechagodzilla from the 70s (there are 3 mechagodzilla movies and they can get confusing) is one of my favorites. It's got robot godzilla, space aliens, ancient ridiculous prophecies, and features the very underrated monster Anguirus (a sort of spiky mutant snapping turtle sort of thing) as one of its supporting members. This is excatly what people think of when they remember growing up in the 80s and 90s and finding Godzilla movies on cable on Saturday afternoons.
Godzilla vs King Ghidorah from the early 90s is just crazy, crazy, crazy fun. Plus Godzilla fights a three-headed cyborg dragon that shoots lightning and there's time travel involved, so there's that going for it. It also is only available as a dub, which ups the cheese factor about 50x.
Godzilla vs Destoroyah has one of my favorite Godzilla suit designs, and has some really cool effects and brutal fights. It's also actually kind of sad at the end, but in a good way. It's also only available as a dub with no subbed JP version DVD, but it's still quite good.
and finally Godzilla, Mothra, King Ghidorah: Giant Monsters All-Out Attack (yes, that's seriously the title. All of it.) is strikingly different as a monster-vs-monster movie that casts Godzilla as the unadulterated bad guy, but it's got some gorgeous visuals and it gives you a wide-cut swath of bits and pieces from the Godzilla history, even if it beats you over the head with the "war is bad" aesop.
I realize you asked for 1 movie and I gave you like 6. But you're basically asking me to pick which of my children I love the most
Also I think which movie is best for you comes down to what you want.
Best overall movie & most surprising movie when held against the image of Godzilla films: Gojira (1954, Japanese cut)
Best classic 60s drive-in monster movie: Godzilla, King of the Monsters (1956 US recut of Gojira)
Most crazy fun: Gozilla vs King Ghidorah (1991)
Biggest monster smackdown w/Godzilla as the bad guy: GMK: GMAAA (I'm not rewriting the whole thing
) (2001)
Best example of the old-school saturday afternoon movie matinee Godzilla: Godzilla vs Mechagodzilla (1974)
Most intense, exciting movie: Godzilla vs Destoroyah (1995)
What most people consider the best movie after the original: Godzilla vs Mothra (1964)
Really, if you know what to expect you can't go wrong with most of them. Just know that when the movie was made impacts the tone a lot. The 50s and 60s movies were often made to be strong sci-fi. The late 60s into the 70s often geared down and were more light fantasy that could appeal to kids. The 80s into the early 90s were often darker and more grown up (vs King Ghidorah and vs Spacegodzilla were crazy trippy, but not necessarily childish) and the 2000s movies were often more serious with a heavier dose of moralizing (except Final Wars, but that was kind of a mess)
Except that under no circumstances would I recommend you watch any of: the 1998 US one with Matthew Broderick, Godzilla's Revenge (aka All Monsters Attack), Godzilla vs Megalon, or Godzilla vs Hedorah (aka Godzilla vs the Smog Monster). All of them are awful in their own unbearable ways.