To be honest I think it would be best to start off with the classical schoolgirl Cracky. Although Nurse Cracky is still fuckin' awesome, the whole red nose/cat ears thing is what Cracky's most known for. Like an Anonymous in this thread said earlier, Nurse Cracky isn't that noticeable because there's tons of sexy nurses, but Cracky's noticeable for the cat ears and shit.
Just sayin'..