Obviously that's a subjective answer, and popular opinion has gone back and forth over the years. Personally I think Vergil is the greatest, but Ovid is my second. Certainly I think the Aeneid, written by Vergil, is assumed by most to be the greatest work of Latin literature, comparable to the Iliad of Homer. So he's got that going for him. One measure of success also is that pretty much all of the Aeneid has survived 2000 years- that's saying a lot. Ask different people who their favorite is and you'll get different answers, but I think most people would recognize that Vergil was the most influential of all Roman poets, the most remembered, and the most celebrated over the years.
So that kind of answers your question about why I think he's better- he's been more influential, more remembered, and more celebrated. Another reason is that he wrote the really only surviving Epic of Latin literature, or at least the only one popularly known. Ovid was great because his stories were wonderful, and wonderfully written, but it can be argued I think that he mostly cobbled together extant stories and put them all in one place. I think that's being very unfair, but it's a criticism you could make. And although Vergil has nods to other writers (most notably Homer) and stories in his work, it's mostly an original piece as far as we can tell.
Bonus: In the first lines of the book, Vergil implicitly states that he's going to outdo Homer by combining the style of the Iliad and the Odyssey into one narrative, shorter AND better than those originals. Pretty gutsy.
Hope this helps