Hmmm after heavily weighing, i'd say Mecha G 2 would win, he may be lousy at up close encounters I mean godzilla tossed him around like a salad and in their first encounter he simply pushed him down. He makes up for it with some pretty durable armor coated in a material designed to absorb and deflect beam attacks, and to make matters more extreme he actually killed godzilla after he combined with garuda. Even without his coating he took a good number of shots from Godzillas Red Spiraled Ray before going down. In terms of overall firepower and durability I'd go with him. Considering this is Kiryu from Godzilla Against Mechagodzilla he'd have his Absolute zero cannon,but the major weakness with that is he has to charge it up and it also takes most of his power supply so if he misses game over, but he's easily the best at melee combat as he went toe to toe with godzilla and was winning at several points. The only other thing in his long ranged conventional arsenal that did any noticeable damage was his maser cannon so in terms of firepower hes not the best but hes pretty darn dangerous up close. The Showa Mecha G is the balanced of the 3 he has decent melee, extremely good firepower and a sturdy shell of space titanium.
They each specialize in the own fields and personally I love all three.
Super Mechagodzilla- Devastating Firepower durability and mobility(lacking in close quarters) Mechagodzilla 2- Devastating Firepower and durability(lacking in close quarters) Kiryu- Strong arsenal, incredible mobility and Great in up close encounters(as issues with taking damage in certain areas as well as a 2 hour time limit and a 1 hit kill weapon that drains most of its power and leaves it vulnerable while charging) Mecha Godzilla 1- Average upclose fighter, Powerful arsenal and some pretty thick armor, it took one heck of a pounding before it gave in.(has issues with his head getting damaged and but later in Terror of Mecha G has that issue taken care of.) Overall Id say mechagodzilla 2 especially if he manages to merge with garuda to go super. MECHAS 4 LIFE
Wow, that was VERY thorough and well written out Sometimes you guys amaze me how deeply you think about this stuff, but on the whole I would have to agree with you. Even though nostalgia makes me tend to prefer the Showa Mecha-G, I think honestly I would support the Heisei model if this scenario was actualized.