Alexander and Madeen are my favourites (Poor Madeen is so rarely used).

I'm not sure what I like about them, FFIX was my first of the series and Alexander vs. Bahamut in that game was my favourite scene back in the day.

I was really happy with the (spoilers) Alexander vs. Odin scenario in ToAU, I'd hope we get something similar being as it's often treated as a 'final' or higher tier summon.