Latin, Italian, or French?

Efficacy: Italian is the way to go here, if you are Spanish speaking. I speak Portuguese myself and it is one of the most natural languages to understand, after Spanish.

Speed: Italian as well, for the same reasons as cited before. It is just so much more like Spanish that will make the faster among the three.

Stepping Stone: Inside the group, Italian as well, it will be much easier to first learn Italian and then learn Latin, they are closer than Spanish and Latin. Outside the group, French will probably give you a broader idea of how a language might work differently from your own. Knowing English in addition to a original mother language gives you a faint idea, but languages in general are very different from one another, in things like structure which are very hard to grasp until you just "know" how to do it.

As for the usefulness, you gotta consider that for the things you said you would be doing, like reading etc, there is just a lot more content in French than there is in Italian. You gotta take in account that not long ago, the global language wasn't English, but French.

Another thing, it sounds like this will be your first language after English, so you WILL feel like it is harder, people use English all the time, we are exposed to it all the time, even people that never studied it end up learning a little bit. With another language, that is mostly not the case, as in, your learning curve will not be aided by a storm of information in that language.