why diablos?
Ummm...well, I like him. Duh. No, but really. I think he's really useful in terms of junctioning and attack (especially in later levels). I mean, you get MUG and ENC-NONE from him, and he can kill Tonberries and a bunch of other big-HP-ed monsters quickly. What more do you need?
Oh, and he is sort of special. I mean, FF8 was my first Final Fantasy. Before that, I had played only like 'Escape from Monkey Island' (hilarious dialogue, btw) and 'Chip's Challenge' (I loved it, even though when Chip died I almost got a heart-attack because the word 'bummer' came so unexpected =P ....I think it was 'bummer').
In FF8 you get the first two Guardian Forces easily. Made me think of them as monsters. Ifrit was a beast that you fought. Siren was inside that horror on top of the communication tower. But Diablos had intellect. Kind of reminds me of Vincent Valentine, as weird as it may sound. When you fight him, he speaks calmly, like an ancient being of sorts. Berates you for waking him up (Vincent, much?). After fighting him he agrees to help you, almost like Rinoa would join your party to 'help you'. It made him seem like another character in the party. I know it sounds weird, but that's what went through my mind when I first fought him.
Later on Bahamut further gives the idea that Guardian Forces are more than beasts, that they are intelligent beings that help you because they want to. I like that about them.
And Dark Messenger just looks too cool to be real.