yo tampoco / a mi tampoco - Me neither
Which ones works best?
2 Answers
A mí tampoco would be used if the sentence or statement you are referring to is formed in a similar way as a "gustar" ("to like") one:
¿A ti no te gustan zanahorias? - You don't like carrots?
A mí tampoco. - Me neither.
Yo tampoco would be used if the subject of the sentence you're referring to is yo, tú, él, etc.
Ella no hizo la tarea. - She didn't do the homework.
Yo tampoco. - Me neither.
Which ones works best?
Best? They are not the same. If the previous sentence has "yo", "el"... then you answer with another pronoun, like "yo". If the previous sentence had "a mí", "a él"..., then you answer with a preposition and a pronoun, like "a mí".
(Yo) No quiero - Yo tampoco
(A mí) no me gusta - A mí tampoco