You, the employer, contribute the most.
You, the employer, contributes the most.
which one would be correct?
This question is too basic; it can be definitively and permanently answered by a single link to a standard internet reference source designed specifically to find that type of information. See the FAQ for guidance on how to improve it.
The part of the sentence offset by commas (the employer) is called an appositive. It gives information about the word "you", but does not take its place as the subject. The word "you" is still the one with which the verb must agree. Hence, the correct choice is the first sentence,