I think it is a mistake to Not levelling up in FF VIII. I levelling up all my characters till 100. And what's more important is I get all GF so I could levelling up my STATS, not only level. I beat Omega easily, using Zell's Booya and Heel Drop, Squal's Renzokuken, Rinoa with 255 status Deffense (Zell and Squall have 255 Status Attack) . And another ability and 100 magics for all kind important magics for all characters in battle. I found FF VIII is easy. But Ultimecia is a bit more hard than Omega, I think. Oh, you have to get Bahamut card! (if I'm not mistaken) it is important since you can refine that card to become Megalixir. and Megalixir is one key to winning. Drinks like holywar or hero drink is useless, I won without that and any invincible moon at all.
FF IX? Mmm, I play it long ago, Can't comment much..
FF-X is more hard than I imagine, I have to play the chocobo, which is a nightmare for me. Do not compare FF-X with FF X-2, I think it is a different game with several same charaters .