PS2 Unleashed is ass, and shouldn't even be considered in this conversation. It's literally a downgraded version of STE. It has a terrible frame rate, no new characters, and less stages than STE. It's awful.
I think Godzilla: STE has better gameplay overall, but G:U has some great additions as well. Better juggle system (back when almost all of the monsters actually had launchers, unlike in G:U). Critical mass makes your monster faster, and the ability to juggle easier, but otherwise, it's not a very interesting mechanic at all. The beam clashing was a lot more forgiving in G:U, which is something I did like. It makes people much more weary when using their beam attacks.
G:U added the much needed parry, and throw escape, which were really great additions to the battle system. Now if only there were a game that offered the best of both worlds..
TechnoForce, I hope you're joking regarding Destoryah. On top of having a glitched Katana Horn, which is broken as skreeonk, he has access to his rage attack from STE, which is a homing meteor that fires from the sky. When it's unleashed, you literally have to keep blocking, because you will get hit. Combine that with the glitched Horn, which builds a ton of meter for Des, and you have what is easily one of best characters in the game. The biggest downside to him is that some characters, including Mothra Larva and Megaguirus, have infinite combos possible on him. (His slow get-up animation makes them possible). Other than that, he's top tier, and definitely better than he was in STE.