I would have to say Yoshimo's speech in chapter 4 deserves the Oscar for best acting. It's great from a roleplaying perspective. He's worth taking along if you plan to replace him/Jan with Imoen.

I also like Korgan's psycopathic rantings. Favorite line: "Me axe be bloody already!"
Runner up: Yoshimo's line "We must be doubly careful here. I'm sure all manner of stupid mousetraps await our toes in the dark."

Worst lines:
Jaheria: "Nature could find a home here if it was properly cleansed and balanced." This sounds a little like the logic of ethnic cleansing, doesn't it? Or destroying the environment and saying, "These woods would be a great place to live if we tore down all the trees and killed all the wildlife in the area." Definitely out of character for a druid or true neutral.

Nalia: "What are we doing trudging around here? Shouldn't we be out helping the less fortunate?" said during attempts to rescue her father at the D'Arness keep. Bad acting.
(However she redeems herself a lot with her statement immediately after the trolls are defeated. A very human response and I was hoping the PC was holding her close, trying to comfort her.)

the best speech is made by the charater when being questioned by the Harpers.

Evil soon to be dead harper: do you have Argessive thoughts?

Character: yes, i'm having one right now. if you could see it you would laugh... well maybe you wouldn't.

the worst... i have to say some of the lovers talks.

its the liralcore (or whatever his name is)
the best one liners in the game, also good spoken
you know once long time ago i was like a moonblade
my brother is a hackmaster +12
or when you converse with him and ask him what his status is
AH SINCE when do you care about me,

hmmm.... the best for me (as always) is the one of MINSC in the final battle... as the music goes to its climax, you nerves are like steel for the final confrontation with Irenicous and this is the final battle ... if Minsc is with you... he will say "EVIL MEET SWORD, SWORD MEET EEEEVVVIIILLLL!!!" man what a combo with the music... its the best line i heard him say ....

The Jaheria romance music is the worst. Very melodramatic. You'd expect that childish sounding music to be more appropriate with the Aerie romance.

Absolute worst: Pick PC sound script #2, the line "Scream, scream for mercy" and hear with what passionate tones your character proclaims this line. Not.
Sounds like he just woke up from a long nap.

I think this is deliberate, he's meant to be Ironic perhaps?. I thought this was cool.

Most annoying line: "You must gather your party before venturing forth"
over and over and... SHUT UP i heard you the first time.

Arak,

You've hit the nail on the head with that one.
I react the same way, yelling "You must shut up!" or something similar but less polite.

The BEST AND FUNNIEST SPEECH I HAVE EVER HEARD IN THE GAME, was in the Umar Hills when the Innkeeper Vincenzo had a little go at his adopted stableboy.

That had me giggling for hours afterwards.