Godzilla Unleashed is the best game in the series, reintroducing famous kaiju and introducing new ones.
This is the monster tier (no order).
Anguirus
Very useful, can jump while rushing, beam attack doesn't do beam fights (good and bad), beam can hurt crystals, high defense, spiky back can hurt enemies.
Baragon
Unique but not perfect, can dig while rushing, beam attack is weak, energy regenerates slowly but automatically, small, good close combat.
Fire Rodan
Hit and run master, can fly, very fast, weak stats.
Godzilla 1954
Same as Godzilla 2000 but with slightly different stats.
Godzilla 90's
Same as Godzilla 2000 but with slightly different stats.
Godzilla 2000
Even balanced.
King Caesar
Weakling, slow energy regeneration, weak attacks, beam attack is weak.
Mothra Larva
Joke character, beam attack is weak, energy regenerates slowly but automatically, small, can dig, can't grab.
Mothra Imago
High tier, fast and deadly, special attacks are very effective, can fly, weak but useful close combat attacks.
Varan
Weak but effective, weak but useful close combat attacks, tricky gliding ability, weak long ranged attacks.
Jet Jaguar
Joke character, can change size (not very useful), weak attacks, strong attacks are slow, weak beam.
Kiryu
Even balanced, can shoot a lot of missiles, can fly, certain attacks are very useful, special attack (Absolute Zero Cannon) is tricky to use but freezes the enemy (only use in 1 one 1 matches).
Mechagodzilla 2
High tier, very powerful, beam attack knocks down enemies, can shoot missiles, can fly.
Mecha-King Ghidorah
Similar to King Ghidorah but has energy shield and paralyzing missiles.
Moguera
High tier, hit and run master, special attacks might be the best in the game, bad close combat skills, can shut down for 5 sec to completely restore energy bar, very fast.
Gigan
Slightly weak but effective, good speed, can teleport across the field.
King Ghidorah
Powerful but pathetic, very slow, can fly.
Mechagodzilla
High tier, similar to Mechagodzilla 2, has a shield that reflects special attacks for 20 sec.
Megalon
High tier, good offense, napalm bombs can easily deplete health bar, can dig.
Orga
Powerful, good offense but is slow.
Biollante
High tier, slowest character, powerful close combat, vines can reach far away (increasing the length of close combat attacks), biggest character, weak defense, can teleport (technically by digging).
Destoroyah
High tier, special attack (Oxygen Destroyer) locks onto an enemy and goes off-screen until it reappears, special attack depletes one health bar, close combat (with laser horn) is possibly the best in the game, powerhouse.
Krystalak
Very useful, fast and aggressive, special attack (Crystal Overload) power depends on amount of energy bars, special attack can be very powerful.
Megaguirus
Fastest character, grab 'steals' foe's beam attack with her grab attack, beam attack can last forever unless it uses last energy cell, one time ability (Hyper mode) doubles the speed of all attacks, can't aim beam attack with remote, can't regenerate energy, can steal energy from foes with tail attacks.
Obsidius
Similar to Orga, immune to lava (Seattle stage).
SpaceGodzilla
Weakling, can float around the stage, can create 2 crystals, crystals can perform two different attacks, regenerates energy slowly, energy regeneration improves when near Spacegodzilla crystals (energy regeneration is still slow).
Titanosaurus
Weakling, can create tornadoes (don't do much).
My thoughts-Megalon, Destroyah, Moguera or Biollante should be one of your main fighters.
Note:Game didn't have Hedorah or Zilla, subtitles are slow in story mode.