Whom did Jacob pay tithes to?

The Torah and the Talmud don't record that information.

The simple meaning of the verse is that Jacob would give his tithes to whomever he found to be worthy and needy. This could include poor people and servants of God. By the time of Jacob, the teachings of Abraham had begun to take root, and there were some servants of God to be found. Also, the righteous disciples of Malki-Tzedek (Genesis ch.14) hadn't yet died out.

Note that the Ibn Ezra commentary (on Genesis ch.28) agrees with this explanation.

The Ramban commentary seems to hint that Jacob was alluding to the future Holy Temple (to which the tithe of yearling sheep would be brought. Leviticus ch.27).

The Midrash says that Jacob fulfilled this vow when he set aside the Tribe of Levi as servants of God.