Are we talking best/worst parts of the plot or the game itself?

Plot

Best: though I think it came out of nowhere, the assassination attempt on Edea. Also, the scene between Squall and Rinoa at the sorceress memorial, and how scary and apocalyptic Lunatic Pandora looks.

Worst: D-District prison, every. single. Laguna dream and that whole concept along with any other flashback, the orphanage realization, and how crappy of a main villain Ulti was. Most of the focus on her was in passing, and her presence wasn't really developed so much as it was merely (poorly) explained, so I as the player never feel any emotion towards her when playing. I want to hate the villain just as much as the protagonists do, and I just can't do that here. We know that she's a psycho bitch and wants to achieve ze time kompression and rule ze world, but we don't know WHY she wants to do that; her motivation is never really explained, and if it's just because she's just evil at heart, I don't buy it.


Game and Mechanics

Best: triple triad, card club, and manipulating the rules (although I don't like the Queen of Cards quest; it's tedious). What separates it from junctioning imo, is that while they're both easily exploitable, triple triad still contains some degree of luck and when Same/Plus is involved, some serious concentration that makes the results more worth it than junctioning. I also love the side quests, especially all of the hidden GF's, the deep sea research center, and PuPu. Oh, and the Ragnarok looks cool.

Worst: every aspect of the dialogue (it's cheesy, excessive, often out-of-place, etc.), the fact that the characters are annoying caricatures of themselves and are wholly two-dimensional (and except for their limit breaks, there's virtually no difference between them in-battle, but that can be said for other installments in the series), the systematic way one can exploit junctioning SO early in the game, which makes the leveling system useless. Most of the music is bad too (I like FH and Eyes on Me though), and there's really no limit on weapon progression (the fact that you can get Lionheart on disc one), and the fact that you absolutely HAVE to have some of GF's for their abilities, yet you never use them in battle.