Just like the title says: what was the thing that gave you the most trouble in FFX? Was it defeating a boss, doing a sidequest (blitzball, chocobo racing, butterfly catching, cactuar village, etc.)?

I had almost no trouble defeating any of the bosses in the game. Even the final boss was relatively easy. But some of the sidequests in this game are a nightmare, especially two: blitzball and chocobo racing. I haven't been able to win a single match against any team whatsoever. Also, the last chocobo race is virtually impossible. My best time is 8.7 seconds and I haven't got near close that record again. I've tried so many times, that I'm about to give up and make Tidus his own weapon with Break Damage Limit using Dark Matters. Some people say that butterfly catching is frustratingly hard, but I could do that just fine.

Dark Aeons by far,you have to train so much to beat them all

and that involves getting abilities on your weapons and armour,which takes way to long WAAAAAAAY TO LONG!!!!

for me, getting used to the sphere grid after playing all the other games with normal levelling up

The obtaining of perfect abilities. Self-explanitory, with being "perfect" they're not exactly going to be a piece of cake to get for your entire party. Not to mention you need a pretty good ability set to actually take on the Dark Aeons and Penance, otherwise... well, you'll never beat them without cheating.

Considering what I said above, no one in their right mind should attempt the Dark Aeons without a darn good set of abilities, so as such shouldn't really fall under the category of "difficult" seeing as they wouldn't be challenged until said abilities are gained. With them, you can't actually lose against the Dark Aeons.

So yeah, the most challenging thing for me was gaining this. 70+ hours of doing JUST that, dammit!

The monster arean and the Dark Aeons were the hardest thing for me

Dark aeons, were that hardest for me 3 down in 7 months 5 more to go

I'd say collecting the primers was the hardest thing for me.