The answer to this question is primarily going to be shaped by your own evaluation of the various speeches in Symposium. Indeed, even before beginning to answer this question, you must first ask: on what grounds will you evaluate which speech is best? Should it be judged on creative grounds, based on its uniqueness or memorability. If so, you might see Aristophanes's speech as a serious contender, along with even the drunken overtures of Alcibiades perhaps. On the other hand, you might judge the speeches based on their philosophical insights and intellectual content, in which case Socrates's speech would probably have the strongest claim.

At the same time, there are interesting questions concerning Plato's original intentions which muddy the waters further. One of the strongest examples of this can be found in Aristophanes's speech. In many respects, this image of one unity of being having been divided in two by the gods, split into halves seeking each other out, can be read as a powerful romantic image (you might call it a soulmate myth, if you would), but there is also something ridiculous in the imagery Aristophanes' employs, as he imagines these original beings as circular in shape, with two faces set in opposite directions, and four arms and four legs. Further complicating this question is Aristophanes's own reputation as a satirist, not to mention the existence of his play, The Clouds, with its brutal depiction of Socrates itself (one which might have played a role in Socrates's later execution). With this context in mind, the question must be asked: in Symposium, is Plato even sympathetic to Aristophanes, or is this an attempt at poetic retribution, ridiculing Aristophanes just as Aristophanes had ridiculed Socrates?

In any case, I think it can certainly be said that the key hinge of Symposium, and the most important of the various speeches within it, belongs to Socrates (as should be perhaps be expected, given Socrates's role throughout the dialogues). For Plato, this speech would have almost certainly been the "best" of the various speeches, as well as the most important of them. However, given the conditions of this particular assignment, the most important judgment will be your own.