IV - Definitely Bahamut. Back when Bahamut was the unquestionable ultimate summon and they didn't have to keep one upping him until he became mid-tier.

V- Don't remember at all

VI - Palidor. Maybe only because I started off most boss fights after I got him using this summon and I didn't typically use most other summons much, but there you go.

VII - Either Alexander, which I didn't even get the first time, or Neo-Bahamut. Knights of the Round is kind of boring overkill and Bahamut Zero is a bit much. Neo-Bahmut gets it just right.

VIII - Definitely Alexander

IX - Probably Bahamut

X - Ditto, but Shiva's pretty badass on this one too.

XII - I hated summons in this one much like I hated the game, so no favorite.

XIII - I stopped very soon after fighting Shiva, so no idea.

XV - I stopped almost immediately after fighting Titan so no idea.