best thing that (has) ever happened to me.

[NAL123]

Somebody says to their best friend,

1) You are the best thing that ever happened to me.

Could they also say:

2) You are the best thing that has ever happened to me.

 

[5jj]

Yes.

 

[NAL123]

Yes.

Do they both mean:

...that (has) ever happened to me until now.

 

[5jj]

No.

The second suggests that.
In the first, the speaker is probably looking back at a past period of their life.

 

[Tarheel]

Do they both mean:

...that (has) ever happened to me until now.

What do you mean by "until now"?

 

[NAL123]

What do you mean by "until now"?

I meant "until the present time".

 

[Tarheel]

I meant "until the present time".

I'm sure that is true.

If I said, "You are the best thing that ever happened to me" the meaning would be clear. However, if I added "until now " the person might wonder what has happened to change things.

You can't add words and not expect to change the meaning of the sentence.