Who is the worst GF in your opinion? As in the least useful?

For me it has to be Siren. I just never found much use for any of her abilities.

Worst one to actually summon though is Carbuncle, but he's not the worst for abilities.

Interesting question. Summon-wise, Siren is absolutely the worst. She does pathetically low non-elemental damage with a chance to add the silence status to enemies, which you won't need when you get her at the beginning of the game (since most enemies use special attacks), and by the latter part of the game it makes more sense to just junction pain to your status-att-j. However, her ability L-Mag Refine is fantastic at the point you get her in the game. L-Mag Refine allows you to refine life magic from items. Incidentally, the Balamb town shop offers the Tent item, which refines into 10 Curagas. Buy 10 Tents to refine them into 100 Curagas, then junction that to the HP of one of your characters (I choose Squall). It will raise their HP from around 500 to 3000. Of course, this completely breaks the beginning of the game but isn't finding exploits a naturally fun part of the game?

In terms of summoning, Siren takes the cake, although Carbuncle becomes useless midway through disc 2 and Tonberry is useless pretty much the instant you get him. In terms of abilities, I would have to go with Shiva simply because her skills get negated pretty quickly and the only unique ability she has is Doom, which is useless, and I-Mag Refine which isn't that great. By the end of the game, most GFs you get from Disc 1 are useless except for their abilities. Shiva is useful for the Fire Cavern but not much else. So if I really had to choose, overall it would probably be her, although she gets used a lot by me in the beginning of the game when I'm too lazy to fight.

I never even found a number of the GFs, but since you can't skip the animations and some of them are incredibly long I came to dread using the longer ones, like Leviathan, which distorts my opinion of some of them. Which is a shame, because I like a lot of them in terms of usefulness.