Mine as well put my two cents
or three on the table. This based off my personal experience with the characters. I'll also explain why I choose that monster for that rank.
SpaceGodzilla - What can I say? This game was pretty much made for this guy, being to KO anyone with medium difficulty (at most) if you really know how to use him. He's a large threat and extremely scary to face with his crystals.
Orga - If you thought Orga was crazy in DAMM, then he's gonna surprise you in this game. He becomes scarier to face with the new abilities and longer range to physical attacks. While speed isn't his cup of tea, he packs a harder punch than Saitama could ever dream of.
Mecha-King Ghidorah - As good as he was in DAMM, MKG becomes faster and a lot deadlier than the previous game. He's one of the very few that can actually be on par with SG, with his shield and tasers.
MOGUERA 2 - Despite a low defense, MOGUERA 2 is fully loaded to make monsters get irritated and flown away by missiles and lasers. So anything that is not MKG must be very careful with their health since MOGUERA 2 can deplete it fast if you're reckless.
Godzilla 2000/Godzilla 1990s - Must I say anything? They are the title character, of course they'll be great!
Gigan- I really wanted to make him an S rank, but his weakness of electricity really hurts since about half the monsters use that. But overall, he's really bulky and takes beam hits well. He's a great monster that will beat the heck out of you if he gets in those hits.
Kiryu - With great weaponry and physical attacks, Kiryu is one of the greats and definitely a force to be reckoned with. His missiles can be really deadly if they get you from behind.
Baragon - One of the best physical fighters in the game, Baragon is beast in the game. The phrase, "Size doesn't matter" fits well with him since he can definitely hold his own really well in the game. His fire can be a really annoyance when consistently using it.
Destoroyah - This demon impressed me far better than he did in DAMM. In that game, I felt other monsters outclassed him... Not the case in this game! He is a very dangerous foe when fighting him and he can put your brain to the test if he starts setting up 'mine traps.'
King Ghidorah - Being massively improved in this game, KG is terrifying opponent that can send chills down your spine. Having the second best attack in the game is something KG boasts, being able to quickly and swiftly defeat opponents. He would be S rank if it wasn't for that meddling MKG who just is too fast for him.
Megalon - Being faster in this game, Megalon is a really great monster and can destroy you easily if he gets the upperhand. However, his defense is something to sneeze at since even a wing flap can push him to the ground. Unlike MKG, his speed isn't enough to neutralize his glaring physical weakness.
Jet Jaguar - A really nice and fun monster to play when you want to just enjoy playing. His physical combat is really good, having the metal defense like Kiryu and MG2 really helped him take hits slightly better. His size changes are what really make him fun, so if you're bored as heck, just use Jet JAGA!
Mothra - With complex moves and weak defenses in Imago form, Mothra really isn't the best. What saves her is the ability to have 'two' life bars, by transforming from larva to imago. While she is fast and you can play strategically with her, she's okay, not the best nor the worst.
MechaGodzilla 2 - What the heck happened?! His defense was severely nerfed and weapons are not as good as DAMM. While he is tank with Gigan and JJ, he struggles with some of the better monsters like Kiryu and SpaceGodzilla. MG2, you're my favorite MG, but c'mon, you're better than this!
Rodan - Spamming is pretty much the only way Rodan does good. Other than that, Rodan is just as, if not worse, than Megalon's defense. So being beaten will be either fast or take long if you're hitting and running. Overall, spammers are gonna spam, and that's what they do best. Other than that, they're meh...
Angurius - While he is good defensively, offense is a little wonky. Like Rodan, you have to spam using three or four attacks and that's it! He struggles with both MechaGodzillas, Orga and to some extent, Jet Jaguar. Unfortunately, poor guy just hasn't been improved and desperately needs it.
Megagurius - Man, it hurts to put my favorite female monster down here. But she just doesn't have the strength to beat monsters that are just that - stronger. Her speed saves her from being absolute worst and I can't seem to find anything good to say. Just underwhelming and it's painful to see...
And that's my list. Sorry it's long. Here's the list by itself:
S: SpaceGodzilla | Orga | MKG | MOGUERA 2
A: G2000 | G1990s | Gigan | Kiryu | Baragon| Destoroyah | KG
C: Mothra | Jet Jaguar
D: MechaGodzilla 2 | Rodan
E: Angurius | Megagurius