I nominate these two fictional presidents:
1. Bill Pullman in Independence Day - Whats greater than a president that has nuts and join the frontline? And against Aliens? His 4th July speech is the best ever!
2. Alan Alda in Canadian Bacon - "I want to say to Prime Minister MacDonald: Surrender her pronto, or we'll level Toronto." Just saw this movie again few days ago and forgot how funny it is
At the same time its almost scary on-topic 10 years later...