I have heard the claim that Spanish is the Romance language closest to Latin.
I have also heard the claim that Portuguese is the Romance language closest to Latin.
And I've heard that Romanian is the Romance language closest to Latin.
I've also heard that Italian is the Romance language closest to Latin.
And I've heard the same claim made about Romansh Ladin.
(I'd love to put all the linguists who made those claims together in a small room and make them come to some sort of agreement.)
Be aware that these are contradictory statements. Obviously, they can't be all correct -- but there could be some truth to all of them. (However, it does reinforce the saying that you shouldn't believe everything you read or hear -- even if what you hear comes straight from professional sources.)
Note that, from all the above statements, one language that's conspicuously absent is French. Does that mean French is the least similar to Latin?
To be fair, we can't discount the possibility that there is/was some variant of some Romance language that was even less similar to Latin, and has either died out, or is just not that well known.
There are also several French descendants (such as creole languages) that are farther from Latin. Do those count as Romance languages? Some people will say yes, some people will say no. It's not a clear-cut answer.
However, since there are no shortages of "This is the language closest to Latin" claims, and I've yet to hear one for French, makes me think that, well, French may be the best fit as an answer to your question -- barring any additional linguistic archaeology.