I don't tend to use them that much, purely because it takes a while to summon them, and by the time you have magic at your disposal they sort of become obsolete. But in terms of design, I liked all of them. The Brothers and Carbuncle were very cute, Doomtrain and Diablos both kick a** and then you have the classics: Bahamut, Leviathan, Shiva and Ifrit. This was probably my favourite Bahamut design and Shiva and Ifrit were second only to X.