In what I consider good qualities of an island, Penang would be the bottom because its basically a big, busy city. Tioman is famous because of the movie, South Pacific, and all the islands on the east side are lovely and beautiful.
I used to like Langkawi but not anymore, its too busy. I first went to Langkawi in 2000 and was basically alone. Arriving by the ferry from Alor Setar, I was offered a taxi ride and declined, i said i would walk. When you leave Kuah town there is no more life and there I was walking east and north to my hotel, carrying a non-rolling suitcase. After five hours of walking, i was ready to just sit down and die from the heat. I gave up. I layed down in the middle of nowhere, nobody around and no cars and no water and no shade, propped myself against the suitcase and said goodbye in prayers, apologizing to all the people I had hurt in this life. About three hours later a policeman drove by and stopped. He came and shook me and asked what the H-LL I was doing there? He got me up and took me to my hotel and i lived. Next day, I rented a Proton for RM10/day and taught myself to drive. On Wednesday nights once a month, restaurants in Kuah town gave away free food as a street party, so I got that too. I went to the duty-free shop and bought a bottle of Vodka and one of Scotch, and went to the north beach and drank myself silly, being the one and only human being on the entire beach. Next day I hiked to 7 Wells, met amazing friends who had never met an american before, and toured round and round in the car, learning and nearly having accidents every few minutes. Next day I walked the long walk through the mangroves to the one and only reason I went to Langkawi in the first place, to see the Barn Thai restaurant. What a joy that was. Before I came to Malaysia, I was in america and met someone who had vacationed in Malaysia and they showed me this brochure of a restaurant, the Barn Thai. I had never seen anything like this and they said, you must go! They were right, I must go! Meantime, then PM Mahathir promised to never let Langkawi grow or be spoiled but that proved to be a lie and today its just not the same.
Eventually, a visitor should see as many as possible, there is no reason to avoid any. I know plenty of people who dont enjoy the solitude of a quiet, tropical island so I cant say whats best. But I would say to someone who does love the tropics, enjoy as much of the wonderful natural beauty here before its all gone.