Hahaha, I remember how frustrated I was on the first boss-fight against Reno on the Sector-7 platform. He casts that weird triangular shield on a character which takes that character out of the fight until you attack your character once with a melee attack.
I don't think I even found out that trick the first time around. I just re-tried him so often until I managed to defeat him before all three of my chars where in the shield ^^;
Also, the first time you get to the world map you've gotta go to that little town and then head down to the Midgar Zolom on a Chocobo, get a short cutscene and then you're meant to walk into the cave that's there on the world map. I didn't spot the cave for like 4 hours. I tried literally everything you could on the world map, rode all over the place and leveled all my materia an entire level subsequently xD I even tried beating the Midgar Zolom because I figured, Sephy did it so maybe I should too? I got really fcking frustrated aswell and then I walked into the cave by accident

Ruby, Emerald, Ultima Weapon. The Wall Boss, several of the Jenova fights. I disagree about VII being the easiest. I didn't play it with a walkthrough to be fair. Maybe that's why you found it so easy? Maybe I just didn't spend enough time leveling up ^^ Who knows

So far XIII is by far the easiest I've played ^^; X had some tough boss encounters, but only a few... VIII was easy. IX & XII wasn't. And I disagree with VII but it was the first game of the sort that I played so maybe it's down to that...