"I will leave" is correct. This is the simple future. As you are speaking you are still present so when you leave is in the future.
"I would leave" has a number of possible interpretations. The most likely is the condtional "I would leave (if something)" This implies that you are not leaving, because the condition hasn't been met.
Are you still at the party? I thought you were bored.
I would leave, but I don't have any money for a taxi. So I have to stay here until my dad picks me up.
("Would" has several other meanings, such as the past tense of "will" in reported speech, and for indicating past habit, but these don't apply to this context.)
I wouldn't use "will" or "would". You are speaking about "right now" so you should use a present tense.
I'm leaving. Goodbye!
The present continuous can be used to describe events that are happening while the speaker is speaking and will continue into the future. The present continuous is the right choice of tense in this context.