I voted Catholic, which is a pretty boring answer as it's almost the same religion on so many levels, but whatever. The Byzantine Catholic answers were even more unoriginal! I am just assuming that if Orthodoxy had not survived, then the Catholic church would have an unrivaled claim to being Christ's True Church.
But if Christianity was out of the picture all together, I would probably go with Judaism for its supreme concept of One God of all nations. Islam would not be an option, as it needs Christianity to whine about to be a religion in the first place.
Actually, it's pretty much dead-even between Orthodox Judaism and Mahayana Buddhism. I would want the notion of One Supreme God, but on the other hand the "greater vehicle" school of Buddhism reaches out to the whole world in a way that the Jews never have. They are too insular; containing God's blessings in one small group. So what I would be looking for is a religion with a monistic principle, but that was a personal deity, with a universal message. Hmmm, so I would be trying to invent Christianity! Maybe Hinduism or Zoroastrianism, so many options!