Here's a little demonstration video. This video displays all the Summons in Final Fantasy VI, which are called Guardian Forces (GFs). The only one I didn't include was Gilgamesh, only because he's completely random and has 4 attacks total. It would have taken me hours and hours to get them to show up. Odin managed to show up though, by luck. If you want to comment on which you like the best, feel free to. Here is a list of the GFs names in the order they are shown:
Quetzacoatl - Thunder Storm
Shiva - Diamond Dust
Ifrit - Hell Fire
Siren - Silent Voice
Brothers - Brotherly Love
Diablos - Dark Messenger
Carbuncle - Ruby Light
Leviathan - Tsunami
Pandemonia - Tornado Zone
Cerberus - Counter Rockets
Alexander - Holy Judgement
Doomtrain - Runaway Train
Bahamut - Mega Flare
Cactuar - 1,0 Needles
Tonberry - Chef's Knife
Eden - Eternal Breath
Odin - Zantesuken
I went in order of how they are listed in the game. Odin is the exception, as he's not a useable GF, he randomly shows up at the start of random battles and OHKOs them. Eden is the most powerful GF of them all, as its attack power can exceed the 9 cap. Only a few other moves in the game can do that (Shockwave Pulser, Kamikaze). But Eden's animation is so long that you'll probably not want to use that as a way of doing damage.
I feel bad for the people who spend the entire game spamming GFs to win fights because they don't understand the junction system. It must be god aweful boring to sit there and wait through these summons just to hope to win a fight. They're cool the first couple times, but that's about all you'll want to watch them. So if you plan on playing the game, do yourself a favor and pay attention to the tutorials unless you WANT the game to be super hard.