If I'm on a hot streak,or just repeatedly killing the same victim,I say "Death becomes you"
When I get back from a cold streak I generally say "Now,what's that they say about Karma?"
Small multikill "Two birds,one stone."
Medium multikill "Efficiency;Noun;Me"
Large-HOLY SWEET MOTHER OF JESUS HUGE! multikill(rare) (Gigabyte voice)"I am become Death,Destroyer of Worlds!" (end Gigabyte voice) [Guess the Gigabyte referece]
Skillful melee kill or kill with artful manouvering "Do the dance,do the DANCE, DO.THE.DAAANCE!"
Possession of big n' powerful weapon or vehicle "I LIKE big guns and I cannot lie..."
Aircraft "Death,in 3D"
Heavy armor (on foot or vehicle) "Built like a TANK!"
Snipe (rare) "Just a little off the top..."
Roadkill (land) "That'll buff right out" (air) "Was that a bird!?"
Reduced to tiny,little,bloody,half-cooked chunks of meat "Mmm...Giblets!"
Denial of objective or Redeemer shot "DEEENIIIEEED!"
Hijack/Skyjack "Hi,I'm Jack!"
Rocket kill "Rocked 'em!"
Those are just off the top of my head!