Heero might be an awesome pilot, but there's only so much that his piloting can do. The limitations of the Leo would prevent him from taking down the Devil Gundam. He might have a chance in Wing Zero, but not so much in a Leo.
Also, if you want ultimate mecha that can be mass produced, then I chose the Centurio series mobile suits. To quote myself in another topic:
Blaze wrote...
MS: Tria/Interpol
Before SD Gundam G Generation DS, this spot belonged to Turn-X. However, both Tria and Interpol can take out Turn X and ∀ Gundam with relative ease. Each one is capable of self-regeneration, since they are completely made-up of Nanites, and as a defense, they have Field Imperium, which blocks any attack the same way an I-Field does. Each one can create a Nanite Sword, which is basically a smaller Muramasa Blaster, from its Nanite Wings, along with a powerful gun, Launcher Geminass. The Launcher Geminass, also made from its Nanite Wings, shoots both Beams and Railgun slugs, which makes Freedom's spamming look weak. It's strongest weapon is Relum Dominatos, aka Moonlight Butterfly Funnels. The strongest version of this attack is Nanite Luminarium, which requires either Tria or Interpol and two other Centurio-class MSs and spreads the effects of the Moonlight Butterfly Funnels over a larger area. If ∀ Gundam and Turn-X can single-handedly take out Devil Gundam and Tria and Interpol can take out both ∀ Gundam and Turn-X, you realize that they are even stronger than the strongest Gundams in existance.
Now, the Centurio-series MSs are MP versions of Tria and Interpol with the only limitation being the number of wings and armor. If I recall correctly, Tria and Interpol have seven Nanite wings, which disintegrate to become their hand-held weapons. The Centurio should have three Nanite Wings, the Centurio Regartus should have five Nanite Wings, and the Centurio Consrare is actually equal with seven Nanite Wings. I believe that the Interpol has the most armor, while Tria may have less armor. After that, it goes in a descending order from Centurio Consrare to Centurio Regartus to Centurio .