These are very clear examples of their respective tenses.
The simple present is used for repeated or ongoing action that probably started in the past and continues into the future.
"You are always nice to me" is describing the ongoing relationship between the two people.
She is always happy.
We are always at home.
The present perfect with "always" is used for action in the past that continues to the present, with no implication about the future.
"You have always been nice to me" is a statement of the relationship up to this point (with the hope or expectation that it will continue, or as an observation that it has changed).
She has always been happy (up to now).
I have always lived in Ohio.
Bruce