Remember how everyone thought it was awesome how Graves got a special line when he hits Nocturne with Smoke Screen? Or how Volibear gets a special line when killing Zilean? Well why hasn't Riot gotten in more of those interactions?
Here's a few examples of ones that could really work:
Ashe hits Tryndamere with ECA - "Cool your head, 'dear'."
Morgana hits Kayle with Soul Shackles - "Share my pain, sister!"
Graves kills Caitlyn - "Looks like I shot the sheriff."
Caitlyn kills Graves - "You fought the law, and the law won."
Caitlyn when any Yordle hits a Yordle Snap Trap - "Got you, you little rascal."
Gangplank kills MF - "Yerr booty be mine now, sweetie!"
Annie kills Brand - "Did you burn out, mister fire man?"
Renekton kills Nasus - "All that knowledge has failed you, brother!"
Nasus kills Renekton - "I give you a merciful death, brother."
There's also a bunch of Shaco gems here: http://na.leagueoflegends.com/board/...8#post22644838
Feel free to add your own suggestions. And don't just go for kill quotes.
A couple of personal ones!
Plagueis kills Leona - *laughs tyrannically* "Dawn has broken!!!"
Plagueis kills Shyvana in Dragon Form - "Hi ni vahzah dovah. Zu'u zok sahrot dovah!" (You (are) not true dragon. I (am) most mighty dragon!)
Plagueis is killed by Solar Flare - "Zu'u unslaad! Zu'u nis oblaan!" (I am immortal! I cannot die!")
Ritz hits Kennen with Sonic Dash - "I thought ninjas were supposed to be FAST!"