I'm torn between four. I voted for Ark because I like the fact that it represented darkness and it's summoning animation was cool. I also like Madeen because it's a holy summon, I like Phoenix because it helped me with the Ozma boss battle (99 Phoenix Downs; it kept reviving me no matter what), and Bahamut because during the Silver Dragon/Kuja boss fights (I dunno what exactly happened- maybe there was a glitch in the game) Garnet kept summoning him. She was able to use other commands but she would first summon Bahamut then heal or something else. I think of Bahamut as my protector in that game now.