In this example, I think leaving out "the" sounds more natural. Be careful, as sometimes "the" is not appropriate, for example you should say "most of the time I go fishing alone", not "the most of the time I go fishing alone" - I think it's quite rare for sentences to have two 'thes' in it

You can use either. Adding the extra "the" is a style choice, and both are fully acceptable grammatically and widely used. To my ear, the extra "the" gives you a little, unstressed syllable between the (quite strong) "love" and the very strong "most", so it adds a little to the poetic rhythm. But very many people leave it out and use just "which do you love most?" https://books.google.com/ngrams/graph?content=love+most%2C+love+the+most&year_start=1800&year_end=2008&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Clove%20most%3B%2Cc0%3B.t1%3B%2Clove%20the%20most%3B%2Cc0