I know this thread is outdated, but it irritates me to see these comments. Since armour and weapons in FFX only have a maximum of four ability slots, it is important to take note of the abilities available when creating a perfect weapon or armour. First of all, the ability "Ribbon" does not make the character Immune to everything. As it's description in FFX say's; Grants Almost Complete Immunity To All Status ailments. The only abilities or negative status ailments it doesn't protect against is "Full-Break", "Curse", "Death", ("Instant-Death").
What I rate are the best armour abilities are, of course with four ability slots would be: "BreakHPLimit", "Ribbon", "Deathproof" and "Curseproof".
These armour abilities May make your character Immune to everything; Except for Shinryu's, "Eraser" ability, (100% Petrification+ Erasion effect), Ultima Buster's, "Contamination" ability, (FullBreak+Zombification+ Poison) and Dark Aeon's, "Instant-Death" e.t.c.
Note, "Curseproof" is not necessarily needed. With Rikku, instead of giving her "Curseproof", I gave her the "Master-Thief" instead. You could have "Auto-Haste" on instead. It would be really useful to have this ability on just in case you die, "Instant-Death". You would therefore save time in having to cast Haste or Hastega again. It's your choice.