It was between Space Godzilla, Destroyer, and Barugon...was almost leaning towards Barugon, but what would likely occur in this battle is Barugon would be un-sure of his opponents.
Remember, he was running from Gamera in their first confrontation. It wasn't until Barugon used his freezing mist that he gathered some courage (that's my little boy--or girl

).
Anyways, I suspect that Barugon would be attacking the other monsters and leveling them with his rainbow ray. But his rainbow ray isn't a machine gun type weapon. It takes time and does re-charge after each attack. Though if anyone were to get too close, they'd be frozen solid by his freezing mist. (Wonder what would happen if they cut off his tongue?)
Personally, I think Barugon would be overwelmed by the survivors. Then it would come down to probably Iris, Legion, Space Godzilla and Destroyer. Iris could only really absorb Space Godzilla's DNA to gain other abilities, but if he were to attempt to try and steal the DNA of Destroyer then well--congrats to our Mutated Gyaos for succeeding in getting micro oxygen running rampent inside your body. You're as good as dead...
Though I don't see how he'd "suck Legion" dry or something. Legion doesn't really have blood. Though Iris's tentacles would prove formiddle for everyone but not even Iris could overcome a threat of Legion, Destroyer, and Space Godzilla...it would be too much and he'd eventually be slain.
Legion is powerful but her shield isn't as powerful as some people give it credit for. Besides, I don't think its designed to stop something like Destroyer's micro-oxygen ray. Space G's corona beam would like swoop around her and attack her from behind. Overwelmed, Legion would be...
So now it comes down to Space Godzilla and Destroyer. Two of Heisei Godzilla's most powerful opponents. Who would conquer? Well, though Space Godzilla does have the superior abilities; Destroyer has more deadly attacks. It takes 4-5 direct hits of Godzilla's spiral blast to penetrate his carapace. Space Godzilla's corona beams are strong, but aren't that powerful. In the end, Space Godzilla could only just hold out for so long before being diced by Destroyer's laser horn. (Two words: Shoulder Crystals.)
Winner:
Destroyer