So, Darius was the first champion I bought when I started playing League two years ago. This skin and the voice over lines perfectly nail the Darius "fantasy".
Particularly good lines:
"AAAARRRRR-- you suck."
"What is it you mastered Yi? Sucking?"
"Oh look-- Amumu -- your coach run out of last picks?"
"Fiddlesticks/Nocturne.... Spooky."
"Hey Mundo, I please you go to the fountain"
"Hey Maokai, I know a court you might be related to!"
"The Dunk Prophecies are true, Darius has arrived."
"I don't see a team, I see coattail passengers."
"I'm in the business of dunking... and business is good."
"That's right, I'm on whaat?"
"When I'm hungry, I turn to Ravenous Hydra"
"The Trinity.... is where dunk meets dunk.... meets dunk"
Lines I would like to see (the general trend is, more things to say to top laners):
Riven Taunt -- something mocking her angst
Rengar Taunt
Jarvan taunt.
I appreciated that taunts were directed mostly at rival laners, junglers (because if they fail a gank you can probably taunt them), and a few at targets (I think there was one for Caitlyn). As part of determining your voice files, I think you should probably get some metrics on champions that Darius players tend to have high interactions with at different phases of the game. I'm unsure to what extent that was done.
One thing you might want to examine is taunt range. Getting in range to use the voice specific taunts can be a hassle. Maybe look at a few games and watch for players who use laugh spams and see what conditions they spam them in, it's usually from a position where they are visible to the enemy and would win an engagement at their present position. Currently the trigger for the specific taunts seems to be just ranged base and the base may be too small.
Edit: Also check the voiceovers for purchased items. Some of these do not seem to trigger, or I don't hear them. When I am 27 kills in and about to buy an infinity edge, it's a very supreme moment so that VO feedback that yes, it's time for the Space Jam is super important.