Yeah, if I were to only run one creature, I would play something that shuffles back in to the deck when it dies. My vote would either be for Serra Avatar or for Worldspine Wurm.
If going with Serra Avatar you can run enchantments like Angelic Chorus and Warstorm Surge to double your life total and deal that damage directly to opponents. You can also fling it with cards like fling, or deal the damage directly through cards like burning anger and soul's fire.
If going with the Worldspine Wurm I would just attack with the creature and sac it at the end step before your turn to get the 3 trampling tokens each turn. Than run cards like overwhelming stampede and triumph of the hordes to close out the game once you have a sufficient board state.
If going with Serra Avatar you can run enchantments like Angelic Chorus and Warstorm Surge to double your life total and deal that damage directly to opponents. You can also fling it with cards like fling, or deal the damage directly through cards like burning anger and soul's fire.
If going with the Worldspine Wurm I would just attack with the creature and sac it at the end step before your turn to get the 3 trampling tokens each turn. Than run cards like overwhelming stampede and triumph of the hordes to close out the game once you have a sufficient board state.