The way Gene Hart goes from future, to present tense, to past tense in the span of 15 seconds (along with me being a huge Flyers/Hart fan) is probably my favorite call ever.
http://www.youtube.com/watch?v=q-feMY5gi1M
Obviously, the do you believe in miracles call, Scully's call of Gibson's home run and the call of Bobby Thompsons shot that won the pennant all rank way up there too...along with Harry Kalas' call of Schmidts 500th.