Amazon. Easy.
The only advantage Fourze has is in distance fighting, and Fourze takes his sweet time swapping and inserting switches, letting Amazon close the gap.

Amazon. He fights dirty. Biter.

Fourze. (Which I still pronounce Fours, Thank you). Because he can adapt. Amazo is good at Guerrilla fighting. Ducking out of the trees,s triking, coming back from another direction. Now, He does this very good. But he only does that very good. Fourze has many more options and can adapt to any given situation. Fire, Rocket, Magnet, Cosmic. Not to mention all the weapon switches.

Amazon hides in the brush, Fourze uses Camera to see him using Thermal, and then attacks using Gatling.

Fight over.

Have you actually watched Amazon? Because I have and I remember a lot more straight-out fighting. Amazon is pretty mobile, yeah, but he also does a lot of grappling and wrestling - all while biting chunks out of his enemies (there are a lot of reasons why I pity any monster that has to fight Amazon in multi-Rider team-ups).

Fourze. His versatility is just too great. Besides, he could always fly up into the cold, soul-sucking vacuum of space and throw Amazon out into it, dooming the poor guy to drift eternally.

Fourze would probably lose most of his limbs and his head by the time he made it to the Stratosphere.

Amazon, as silly as he looks, wins this one. Amazon is a brutal fighter while Gentaro wants to befriend even the monsters he fights. Maybe if Amazon killed Gentaro's space goo girlfriend (sending Fourze into orange rage mode) this could be an even match.

Orange rage mode?

Anyway, this is a tough one, but I think I have to vote for Amazon.

Have you seen Amazon fight? He uses guerrilla tactics, but he's also a fighter, something Fourze isn't.
He's not just gonna hide in a bush in the middle of the fight.

Hey, remember the time when Kengo got stuck on the moon and Fourze, with the Machine Massigler AND the Power Dizer's tower mode launching him couldn't even make statosphere?
His jetpack is pitiful. He'd probably lift Amazon a meter before they both drop to the ground.

Yes, but Cosmic States Limit Break actually has him teleport into space. Besides, he never tried that with Rocket States.