Wow - this is a tough question, yet one I've debated before. My first thought is that I would want to be happy. Who in history was happy? I think a truly happy person is one who accepts him/herself, who trusts in good, who loves fully, who appreciates beauty. A complete lack of judgment would be necessary too - how can anyone be happy if there is judgment? Someone else's life, partner, career, possessions, family... all would be subject to judgment.
I think my choice would be either Mother Theresa or Buddha. Both were ruled by love and compassion. Both accepted life on life's terms. Mother Theresa devoted her life to serving the poor; she loved unconditionally in extraordinarily difficult circumstances. Siddhartha Gautama, or the Buddha, devoted his life to sharing and teaching his philosophy and enlightenment; he tried to show anyone who wanted to learn, how to attain peace.
All things considered, and with great respect to many other historical figures (my favorite being Wolfgang Amadeus Mozart, but sadly he was not a happy man) my choice would have to be Mother Theresa. I imagine her to have been completely satisfied with her chosen life, never seeking anything for herself. I doubt that she would have wanted to be anyone but who she was, an example of human love triumphing over all the sadness the world can give. I picture her in the next world, fully evolved, able to enjoy eternity in peace.