Wow!  Getting a lot of attention and a lot of great suggestions in comments!  Will add in a list of all champions and include my favorites from comments as soon as I get time (it's coming, don't worry).  Also, if I don't include yours or don't include your name (I probably won't include names, because I have seen rather similar ones, and I'll probably be blending a lot of them together anyways), please don't feel sad.  I saw your comment, and I probably liked it, it's just I felt that someone else's was more creative or fit the spot better.  Thanks!

(that was long)

Ok.  I can't remember where I saw this concept before, but I feel it's worth bringing up.  

What if champions got a unique global quote activated upon receiving a pentakill?

If you think about it, I can see absolutely no reason why this is a bad idea.  It makes the pentakill experience even more fun, while not interfering with gameplay.

Essentially, it would be like this.  Champion A has got a quadra kill!  Champion A has got a penta kill!  *Champion A speaks a quote that is heard by all players*

Some example quotes would be:

Nasus: "Rest in peace" (an homage to his voice actor)

Sona: Plays a part of "We are the Champions," or other appropriate song (IN ELECTRIC GUITAR)

Gangplank: Reference to the "You will remember this day as the day you almost caught Captain Jack Sparrow" line from Pirates of the Caribbean, albeit appropriate for a situation where you have killed everyone on the enemy team.

Xin Zhao: "Somehow I'll make a man out of you" (in appropriate condescending, Li Shang voice)

Pantheon: "THIS IS SPARTA"

Darius: Basketball end game buzzer.  

Soraka: I weep for the fallen.

Viktor: You could use an upgrade.

Diana: The blind are silenced.

Lissandra: Frozen in an avalanche.

Ok, as you may have guessed, this is an obvious excuse to put awesome movie lines into League.  However, I still think it'd be cool to get a pentakill with Pantheon and hear him say, "THIS IS SPARTA!"  Thoughts?  Suggestions?