The American Revolutionary War, the odds of winning that war were slim. On several occasions it could be argued that it was almost lost.  The surrender of cornwallis needn't have ended the war, merely prolonged it if England wished.

Many of these other wars throughout history, were inevitable.  i.e. the two biggest kids on the block were always destined to fight and history was always going to be affected and this type of conflict is inevitable. The magnitude of the outcome of the revolutionary war combined with its improbability give it the status of most important war.