The two sentences are quite different. The second one has a very odd meaning.

“I wouldn’t be lying if I said…” means “It is true that…”, so the first sentence means, “It is true that this is the best evening of my life!”

On the other hand, “I wouldn’t lie if [condition]” means “I might lie if the condition isn’t met.” In your example it means, “If I don’t say …, I might lie (after).” It means that saying the words “This is the best evening of my life!” somehow prevents you from telling lies, like it’s a magical incantation.