I don't think any of them are very hard to obtain... but considering where you are in the game when you get the item that summons Diablo, and how you ahve to defeat him... I'd say he would likely be the most difficult to obtain... unless you know how to junction yourself well from the start, which makes him just a simple boss fight.

That said, the most complicated to obtain would probably be the Brothers, if only becasue of all the things you have to do to get to fight them... but even that isn't too hard.

If you have been utilizing junctions properly, and have allowed yourself to level up a lot, then Eden can be pretty damn hard to get, becasue Ultima Weapon would be more difficult to defeat... but yeah... if you know how to play the game, none of them are really hard to get.

The hardest GF, for me to obtain, was Tonberry King. I swear to God, I hated it, mainly because I fought through 3423423 Tonberries, to get to the little idiot! and then I am, alas, KO'ed by the monster. And so, after swearing alot, I had to restart and fight through another batch to get the chance to own the little booger. xD

Wow... you should run into him after only 20 or so tonberries... or are you just exaggerating? Cause I've told you a million times not to exaggerate.

You have? Opps! Yeah, I was. But seriously, yes, I had the worst time imaginable with the little green devils. But you know what? The next go around I owned them all, even the king. Ha ha, don't piss me off. I'll have revenge. Humm, I'd say the second hardest GF, for me, was the big D. Diablos, that is. I think it was because I tried to fight him way too early. My levels were low and my junctioning was terrible.

I don't think there is an harder GF to obtain, but perhaps Doomtrain because i need to spend some time getting the items.

Probably Cactuar, that twat wouldn't stop running away.

Errr, Tonberry King was the hardest.
I hated that one-hit KO thing