@JLeslie From the OP’s question it is not really clear whether he has already studied Latin or if he is considering studying it. But, one could assume that he already knows Latin and he wants to know which of the Romance languages mentioned is the most similar to Latin, so that he can use his knowledge of Latin to grasp and acquire the new language faster and easier.
I speak Spanish and when I see or hear Latin it is not familiar to me. I don’t know why that is because it is very obvious that a lot of words are very similar to their Spanish equivalents. For example look at these lines in Latin:
In nòmine Patris, et Fìlii, et Spìritus Sancti. Gràtia Dòmini nostri Jesu Christi, et càritas Dei, et communicàtio Sancti Spìritus sit cum òmnibus vobis. Et cum spiritu tuo. Gratia vobis et pax a Deo Patre nostro et Dominio Jesu Christo. Benedictus Deus et PaterDomini nostri Jesu Christi. Et cum spirtu tuo..
Someone who knows Spanish should be able to guess what most of them mean.