Alexander is my all time favorite summon in Final Fantasy 8. The healing power and magic boost has always helped me when playing. Also the medical items potency is increased which helps when it's time for attacking or defending.
The dragon beast Bahamuts is just my favorite in the game because of its strength as a beast of fire. Even though Bahamuts wouldn't be considered the strongest but it's a character of magical creature and I love anything that is magic related.