I think we�ve already met. (this is present perfect simple because you refer to the experience you have had in your life up to the moment you say it.)
I don�t think we�ve met (before). this is the same as the first one.
This is � / Meet � This is imperative. You make somebody do something. May be it is easier to understand if you remember the phrase in every action film: " Go! Go!" then the verb is used in the 1st form.
Have you met �? This is the same as the first one again.
Yes, I have. / No, I haven�t.
Yes, I think I have.
No, I don�t think I have.
Hello, � (name)
Nice to meet you. (informal) This time it is infinitive as there is an adjective "Nice" that is before the verb "meet"
Pleased to meet you.the same as nice to meet you.
Which is alright?
Hope you find it useful:)