I would choose Avenger of Zendikar. Large free-for-all multiplayer games tend to have lots of politics and last a long time. You'll probably have time enough to get Avenger of Zendikar out and take advantage of it abilities. Based on how many powerful the landfall ability is in this block, you'll probably have a number of other things to work well with its ability.

Since you'll be facing multiple opponents, you should expect at least one of them to have spot removal. Avenger of Zendikar gives you a bunch of creatures instead of just one. If Sphinx of Lost Truths gets blown away, all you have to show for your 7 (assuming you kicked it) mana is 3 cards.

[[Avenger of Zendikar]]