I might also want to be friends with Simon, like the post above. Simon is an intriguing person, supportive and imaginative.
Exploring the island boldly and quietly, Simon keeps his own council and I would probably enjoy picking his brain a bit and finding out what he is thinking.
Also, it's interesting to me that Simon may be the person who needs a friend the least or the person who needs a friend the most. There is some ambivalence there.
I think I would want to befriend Simon most of all. He isn't the leader and he isn't going to necessarily make anything more fun or more exciting. But he has a very different outlook on things and is obviously really caring and insightful, but he does sort of keep to himself as well, doesn't seem like the guy that easily just makes friends, etc.
Of course with the hindsight of getting to the end of the book and knowing that my friend would be brutally murdered by the boys, that might make it a bit harder to choose him, but I might still do it. I just find his persona to be pretty fascinating and I would want to get to know him more than any other boy.
In making your choice of characters from Lord of the Flies, consider the personality traits of the main characters that are appealing, but also consider the fact that you are going to be on the island with these boys for a good while. So, while the golden boy Ralph is appealing for his charismatic looks and his athleticism, and Piggy for his intelligence, there may be someone who is a bit mysterious and more of a challenge to know. He also will be more likely not to be surrounded by others so that you could get to be good friends with him--that is, if you, too, do not prefer crowds.
Ralph is chosen as the leader and, thus, has people around him. He also is faced with having to make decisions, some of which you may not agree with. Piggy is asthmatic and overweight, so he cannot engage in some of the physical activities that the other boys enjoy. He is not popular because of his physical shortcomings, and will be, as he is in one of the later chapters, on the outside of the circle. Jack is bold and wild, perhaps too much, and, thus, runs the risk of getting carried away as he does with the masks and hunt for the pigs. Simon is kind, always helping Ralph when the others follow Jack; he also is very intuitive and deep. Thus, being friends with him may be interesting and informative. He, like Piggy, remains true to himself throughout the narrative. And, perhaps, if he had a friend to support him and explain what he has tried to tell the others about the beast, Simon would not have to meet the misfortune that he does.
This is such a personal choice, and I'm sure your teacher isn't particularly interested in who I might want to be friends with. Who you are is going to determine your answer, and I hope any opinion(s) you get here will simply motivate you to think about your own answer to the question.
That being said, I'm pretty sure the character I would most want to befriend in Lord of the Flies is Piggy. I, too, like to make lists and maintain order, and I connect to his idea that rules should be followed. As a younger, less mature person, I might have joined the group and made him the butt of all jokes; however, knowing who I am, he's the closest to being friend material for me. From the first page to his last breath, he stayed to true to himself despite the ridicule and mocking, a trait I admire.
I think that at the start of the book I would want to be friends with Ralph. I hope that at the end of the book I would still want to be his friend, but I think I would be less in awe of him than I would have been at first.
When we meet Ralph in Chapter 1, he seems so cool. He sounds like he's handsome and has a good body. He seems like he's in control. He's athletic, he's confident. You can see how Piggy immediately looks up to him and I think I probably would have too. So at the start, I would have wanted to be his friend, though I think I might have been more of a follower than a friend.
By the end of the book, it would be more clear that he was not perfect. At times he ends up looking pretty weak. So I think I would not really look up to him so much anymore. But I hope I would still like him because he was certainly someone who was trying to do what was best and he was certainly much more appealing than Jack and his gang.