I love all the summons in VIII as I think they have some beautiful animation.

Shiva in her VIII form is my favourite.
In terms of usefulness; as others have said Cerberus and Doomtrain.

I think my favourites in terms of their little sequence are Shiva, Bahamut and Cerberus. I also love Carbuncle as it's so cute.

Eden is super powerful but my GOD that animation goes on forever.