The Psycho Gundam from Zeta Gundam, The Psycho Gundam MK-II from Zeta and ZZ Gundam, and the Destroy from Gundam Seed Destiny are the Largest Gundams or Mobile Armors if you will. Id say the Devil Gundam from G gundam.In its full form it was larger than a space colony. That wasn't its "true" form though. The final form Domon faced inside is deemed its true form.