Well here we go then.
The Divine Saiyan who was once mortal but with constant training and fighting with powerful beings became the strongest saiyan. (It is not that hard to be the strongest when there is less then dozen of you but still) Rivalry with Vegeta really pushed him to becoming stronger. And any new enemy he faces was nearly always stronger then Goku so he over time learned that he needs to train like a lot and get stronger to the point where he even attained God Chi and became kind of Demi-god/Divine Saiyan if you don’t mind calling him that.
Powers and abilities: Incredible tactical intelligence and fighting genius. Overall besides fighting average intelligence. (Goku is not dumb. He is just childish sometimes and his lust for food and fighting sometimes make him look like that). His power level was always expanding and breaking the limiters and getting to new heights. When he started he was powerful enough blow up cars. Now at peak of his power he is strong enough to shake the universe with his power.
Weaknesses: Likes to drag on fights sometimes and wants his opponent to go all out so he can have a challenge. Has pure heart so he will not resort to killing unless absolutely necessary and this costed him his life and sometimes even fate of the planet Earth and humanity itself. Can’t breathe in space still.. even though he has god chi. And he might be able to survive in vacuum in space for some time.
Average mortal on first glimpse but there is nothing average about his power level. He was simple mortal with average life like any of us until it came the day where he got fed up with his average and when he saw a monster trying to kill a kid. He saved the kid but monster didn’t want to back down. Saitama got angry and said that he is tired of his life and he will want to become the hero who can punch rotten villains away with just one punch. He defeated the monster after hard battle. There he started training and after 3 years he became so powerful he became bored because he realized that having overwhelming power is just boring and lame. (Think of it this way. You play some game and enemies are powerful they give you trouble your health reaches critical level and you win. It was worth it you cheer you are happy! Now Use console commands and type God mode or something similar. It will be fun first few times. Imagine now playing same game over and over with God mode. You will get bored eventually. Same happened for Saitama)
Powers and Abilities: Overwhelming power level to the point where non of the enemies posed even a little threat and 99% of them got killed with one punch. His willpower is enormous. He will never back down from being pure hero and never will succumb to evil. Will always fight and be hero no matter what. Even if everyone are against him he will stand for right cause still. The peak of his power what he demonstrated so far is jumping from Moon to Earth in manner of few seconds. And he can punch away energy wave/beam that can destroy the Planet. Still all this is not even a glimpse of his true power apparently.
Weaknesses: Will not use full power right at the beginning of the fight because even that small percent is capable of killing most creatures in one punch. Usually doesn’t take fights seriously. Although if enemy does somehow manage to survive a punch then he will drag on the fight to see how powerful enemy really is. Also will not kill humans. We saw him only killing monsters. Can’t breath in space although he can hold breath for extended period of time most likely way more then any human since has just chilling on the Moon. Also can survive vacuum of space for extended period as well. And yeah he can’t fly because humans can’t fly although he can maneuver in air with relative easy to dodge the projectiles or energy beams.
Well overall depends on the 3 factors.
First which version of Goku are we using? Dragon Ball Goku, Dragon Ball Z Buu Saga Goku or Current Goku, OR Dragon Ball GT Goku, Although that one is not canon so I will not use GT in this battle
Second what is limit of Saitamas power level. Are we gonna assume he is at Star Level buster maximum or he has infinite strength/speed/durability in 3 dimensional space-time.
Third their moral? If they ever faced in battle they would never kill each other. Goku will not kill since he will see Saitama is actually kind at heart and good guy and Saitama will see that too. Also Saitama will think Goku is just powerful human and Goku might think Saitama is another unknown Saiyan or maybe powerful human. For the sake of battle both will have morals off and will kill each other if capable.
Dragon Ball Goku after he defeated King Piccolo vs Star Level Saitama - Here Goku was around small country level since he is equal or slightly stronger then King Piccolo. Saitama probably doesn’t one punch but after consecutive normal punches Goku is blown to pieces.
Dragon Ball Goku after he defeated King Piccolo vs Infinite level Saitama - One punch!!! And Goku is dead.
Dragon Ball Z Buu Saga Goku vs Star Level Saitama - Here Goku at maximum with Super Saiyan 3 was roughly equal to Kid Buu which means he is Solar System Buster. Saitama met his match and must fight seriously. He is enjoying it and Super Saiyan 2 Goku is struggling a bit. Goku goes Super Saiyan 3 and Saitama is overwhelmed. Goku uses Instant Kamehameha to vaporize Saitama from behind.
Dragon Ball Z Buu Saga Goku vs Infinite Saitama - One Punch!!! And Goku survives but is badly injured. He eats a Senzu Bean and gets up. Powers up to Super Saiyan 2. But it is not enough. Saitama is easily blocking all attacks. Goku realizes and goes Super Saiyan 3 where Saitama is surprised bu sudden power boost. Saitama gets ragdolled around. He stands up and uses consecutive normal punches where Goku hardly blocks and gets hit a few times. He is injured but can go on. He powers up a bit more and uses full power Kamehameha. Saitama gets slightly serious and uses Serious Punch. Kamehameha is blown away back to Goku. Goku survives but is badly injured. Goku stands up and flies to the sky saying *People lend me your energy!*. Saitama watches and thinks *Whathe doing?* Saitama gets bored of watching, jumps to Goku and Punches him. Goku is dead. NOTE: Solar Flare wouldn’t give Goku too much advantage since he can’t hurt this Saitama. Also Spirit Bomb is useless against Saitama since he also has pure heart.
Current Goku with Full power Ultra Instinct vs Star Level Saitama-Base Form Goku gets excited at how poweful Saitama is for a human. He even mentions that Saitama is more poweful then Krillin but lacks his martial art training. In the end Goku slaps Saitama and kills him.
Current Goku vs Infinite Saitama - Base form Goku is fighting with Semi-serious Saitama and realizes how poweful Saitama is. He powers up to Super Saiyan. Saitama is still unfazed. Goku Powers up to Super Saiyan 2. Saitama still unfazed but power up uses Consecutive Normal Punches and injures Goku a bit. Goku gets up calms his mind and turns Super Saiyan Blue. Here Saitama gets slightly serious and fights with Goku. They seem equal punch after punch but Saitama uses Serious Punch and Goku gets mortally wounded. Goku uses Senzu and gets up. Realizing that was last senzu he powers up to *Mastered* Ultra Instinct. Saitama throws punches Goku evades them all and punches Saitama back. Saitama excited at how powerful Goku is he smiles. He says *Now I will fight seriously*. Goku and Saitama exchange blows and Goku is surprised how Saitama can fight him on equal grounds. Saitama is starting to use more and more of his strength and Goku seems to be more and more overwhelmed. Goku uses all his power on Instant Full Power Kamehameha wave.