Spanish. English is not a Romance language, despite the influence from such.

(Edit):

You are at a great advantage knowing both languages and learning French, as it is etymologically related to Spanish and historically related to English. General grammar rules and a good amount of vocabulary from Spanish will be the bulk of what you’ll see in French. English does have a large amount of vocabulary stock descended from Old French and Norman, so you’ll have an even greater advantage than somebody who speaks, say, Russian.

Spanish for sure, it is in fact quite similar!

I agree with Yogi Castaneda's very accurate explanation except for the relevance of Old French when it comes to understanding modern English... he's not actually wrong about this, but I reckon this relevance is really not that helpful in the learning process.

On the other hand, modern French is full of borrowed English phrases, so that should definitely help!