A list of Godzilla series kaiju I think are the most powerful.
all other versions of Godzilla
#10- KING GHIDORAH
King Ghidorah is Godzilla's most famous foe, and one of the most powerful. He is 100 meters, 150 meters wings, & 30,000 tons in the Showa series, and 140 meters tall, 175 meters wings, & 70,000 tons in the Heisei series. Both versions can fly a mach 3. Ghidorah's main weapons are his gravity beams, which he can rapid-fire from all three heads. These lightning-like bolts can destroy skyscrapers & hurt Godzilla. In addition, he can create hurricane force winds with his wings. King Ghidorah durability varies: sometimes he has easily withstood Godzilla's ray, other times it badly damages him. In the Showa series he could travel thru space in a magnetic meteor. In the Heisei series he could constrict foes with his necks.
Biollante's powerful final form is 120 meters tall & weighs 200,000 tons. This makes her the heaviest Toho kaiju ever. Biollante can shoot radioactive acid sap from her mouth and vines. Her vines/tendrils have sharp toothy mouths at the ends that can pierce Godzilla's flesh. The vines can also constrict her foes. Biollante possesses great physical power due to her size, but her sheer mass & plantlike nature make her quite slow-moving. Her huge mouth is full of sharp teeth. Probably her greatest power is that she is practically immortal. In addition to regeneration (Godzilla blew a huge chunk out of her head that healed very rapidly) she will turn into golden spores when mortally wounded & reform later.
#8- MECHAGODZILLA 1
The showa series Mechagodzilla is 50 meters tall & 40,000 tons. He can fire beams from his eyes that equal Godzilla's ray. If he spins his head rapidly, a force field will be created that can block attacks & shock any monster who touches it. MG1 has a lightning-like beam from his chest that sliced through a mountain peak with ease. He can fly at mach 5 with the jets in his feet, which is faster than any other showa kaiju. His strength matches Showa Godzilla's & he is covered in space titanium armor. MG1 can also shoot powerful missiles from his fingers, feet, throat, knees, and toes.
#7- MONSTER X/KEIZER GHIDORAH
Monster X stands 120 meters tall & is 60,000 tons. As Godzilla's 'last' enemy, he was made to be a formidable foe for the king of monsters. Monster X can shoot gravity beams from his 6 eyes that are equal in power to Godzilla's ray. He is as strong as the Final Wars Godzilla. He can withstand Godzilla's radioactive breath with little damage. Monster X is also a skilled fighter and jumper. He apparently has a limited form of levatation, and can transform into the more powerful Keizer Ghidorah. In this form he is 140 meters tall, with 150 meter wingspan, and weighs 100,000 tons. Keizer G's gravity beams are stronger than Godzilla's ray, and have anti-gravity/telekinetic powers, enabling him to throw Godzilla around with ease. His bite can drain an opponent's life force. Keizer Ghidorah's main weakness is durability: a single gravity bolt or atomic ray can blow off one of his heads.
#6- GODZILLA 2004
This version of Godzilla appeared in Godzilla: Final Wars. He is 100 meters tall & 55,000 tons. Like all Godzillas, he can shoot an atomic ray from his mouth. Godzilla 2004's beam is strong enough to hit a meteor in the upper atmosphere and destroy it. He is durable enough to survive the resulting explosion with no damage, even though the blast destroyed Tokyo. He is strong enough to judo throw the 100,000 ton Keizer Ghidorah. Godzilla 2004 can also jump well, and has the regenerative powers that most Godzilla's posses. After being surged with keizer mutant Ozaki's energy, he gained the power to shoot a red hyper spiral ray that can push 100,000 tons into space & completely destroy it. Plus, he can create a surge of energy through his touch that damages those grabbing him.
Hedorah's strongest form is his final form, which is 60 meters tall & 48,000 tons. Hedorah is the most powerful kaiju in the Showa series. He can shoot a crimson beam from his eyes that burns Godzilla's flesh. His sludge body is impervious to any form of attack except electricity & intense heat. He's also highly corrosive: the flesh on Godzilla's hand melted off after the monster king punched through Hedorah. He can ooze huge amounts of sludge, or fire small bits of it, that are also capable of injuring Godzilla. Hedorah's sulfuric acid mist can suffocate anything. He can grow more powerful & acidic by consuming pollution, jump great distances, and turn into a pancake shaped form that flies at mach 1.
#4- MECHAGODZILLA 2
The Heisei Mechagodzilla is 120 meters tall & weighs 150,000 tons. He flies at mach 1, and has a great arsenal of weapons. The Mega Buster mouth beam is as strong as Godzilla's atomic ray. He can shoot paralyzing missiles from the holes in his shoulders, as well as beams from the eyes. MG2's armor is Super Heat Shield NT-20, which means that the atomic ray can't damage him, and he draws power from it. The power is used to shoot a plasma grenade attack from his chest that's strong enough to injure Godzilla & kill Rodan. The shock anchor cables shot from his wrists can discharge electricity, and were used to kill Godzilla by hitting his '2nd brain'. The flying ship Garuda can attach to MG2's back, and increases flight speed to mach 2 as well as adding strong laser cannons to his arsenal.
#3- GODZILLA (late Heisei series)
This incarnation is 100 meters tall & 60,000 tons. His main weapons are the atomic ray which all Godzilla's possess, and a powerful omni-directional blast of energy called the nuclear pulse. His strength is the greatest of any Godzilla's (he can lift 150,000 tons) though he isn't as good a close range fighter as most Godzillas. He can, of course regenerate. 1990s Godzilla can redirect electricity (he fried MechaG 2's circuits the 1st time they fought). He can absorb additional energy for greater strength. By far Godzilla's best power is the spiral fire ray which he got from absorbing Rodan's energy. This red beam is strong enough to destroy Mechagodzilla 2 or Spacegodzilla with just a few shots, though Destroyah wasn't killed by it. He used this beam all the time while in Burning form, and when he melted down he unleashed a spiral inferno ray strong enough to cripple Destroyah with just a glancing hit.
Destroyah is 120 meters tall, 230 meters long, has a 210 meter wingspan, and weighs 80,000 tons in final form. This giant crustacean monster can fire a micro-oxygen beam from his mouth that is powerful enough to topple & harm Godzilla. While not agile, he is very strong. He can overpower Godzilla, and lift him with his pincer tail. The micro-oxygen monster can withstand Godzilla's spiral fire ray with some damage. He can split into many aggregates 1/6 of his full size. Upon reforming, he has regenerated all wounds. Destroyah's greatest weapon is the laser horn, a slicing arc of energy that can cut Godzilla's flesh like butter. It also neutralizes regenerative powers. He is highly vulnerable to cold, and was slain by freezing weapons after being crippled by the spiral inferno ray.
The cosmic powered clone of Godzilla is 120 meters tall, and weighs 80,000 tons. He is the most powerful kaiju in the Godzilla series. Spacegodzilla's main weapon is the corona beam from his mouth that homes in on foes & is powerful enough to pierce Godzilla's hide. He can create a gravity tornado that can send Godzilla flying across Tokyo. He can turn into a crystal encased flying form for traveling across planets or through space. He can cause giant crystals to sprout from the ground, which draw power from space that he can use. Spacegodzilla possesses telekinetic powers that he uses to shoot crystals at foes and to levitate. He can create a force field that can block Godzilla's atomic ray. He has the regeneration of Godzilla, and enough physical power to stab MOGERA with his tail and fling the 160,000 ton robot. He can also shoot EMP orbs, discharge energy through touch, and fire short-range bolts from his shoulder crystals. The only way to beat Spacegodzilla is to destroy his crystal towers & shoulder crystals, leaving him with less power.