Jecht is hard. My first playthrough I pretty much ignored side content and had no celestial stuff. I did beat Omega Weapon at the ruins and had to retry Jecht a number of times. In later runs I one shot him with Attack Reels, but he's hard if you don't have all that stuff.
I think Ultimecia is the hardest. She can still wipe you if you don't react on those 1 hp spells. Esp. Griever was a nasty fuck during my first run.
Yeah, Ultimecia was just this huge gauntlet that wasn't really matched in future FF matches(Even previous ones)
First there is a challenge that she randomly picks the party she faces. Meaning if some characters are underleveled, or not equipped with GFs, then it can end in a quick wipe if not careful.
Second, Ultimecia herself deals offensive spells that tend to inflict status ailments. Maelstrom is a pain to go against. She'll cast high level spells like Bio and cause status ailments up the wazoo.
After her is Griever. He deals AoE damage as well. He'll cast Lvl ? Death which can wipe your party if their levels are a multiple of 5, as well as use Doom which would kill your character after a number of turns.. He'll also cast Shockwave Pulsar which would do massive damage.
After that Ultimecia junctions herself into Griever. In this form she uses the highest level magic that does aoe. Tornado, Earthquake, her finisher is Great Attractor which again, does huge aoe damage. If you somehow can live through this phase, you go into her final form.
Her final Form is a completely mindfuck. It's design is absolutely amazing. Not only that, she uses Hells Judgement which reduces everyones life to 1. halfway through the fight, she gains access to her body in the lower abdomen, and she can use Apocalypse which will do amazing damage on you. All the while she is casting high level spells doing huge damage.
Unlike Sepiroth, there is no scripted 'hit them once and the game ends', FFVIII ends with you struggling over and over and over again. There is no break point between fights, it's do the gauntlet of all 4 forms one after another. The theme of fighting against time, not being chained to fight are made reality in this boss battle-because you really are struggling against time as you keep fighting over and over and over. As a kid, it was a matter of playing this boss and trying to beat it before you had to go to bed or had to go out. Ultimecia demanded your attention.
Man, Ultimecia is such a great boss fight.