Thank you all for your replies.
After reading your answers (each one several times...) and discussing it with three more people my conclusion is as the following:
In both sentences (Meeting you was/ is the best thing that has ever happened to me) it is clear that the meeting event took place in the past.
However this event is connected to the present, it was and still is (=present perfect) the best thing that….
Maybe, if you think about it like this, MW, then it will make more sense. The differences are, to my mind, not to be found in the grammatical structure. We use language to effect meaning and emotion.
1. Using "Meeting you has been the best thing ..." shows the greatest intensity, as the present perfect illustrates greater importance.
2. Using "Meeting you is the best thing ..." shows a slightly lower level of intensity; the present tense shows it still is the "thing".
3. Using "Meeting you was the best thing ..." shows the lowest level of intensity; focus on the particular past action of meeting.
Now, of course, intonation can make any one of them seem much higher or lower.
Therefore it should be changed to:
Meeting you has been the best thing that has ever happened to me.
It seems to me that the present perfect is necessary here with both verbs because both events connect the past with the present.
But remember, and this is CRUCIAL to understanding this use of the present perfect. There is nothing compelling us to use this present perfect form. It relates to how intense we want to make our language.
The "meeting event"= was and still is
The "happening event" by the expression ever (from a certain point in the past till now).
Or may be I'm just totally

?
Not at all. That was a well thought out, excellent analysis.