Final Fantasy XI after the release of Final Fantasy XIV. Not that the two games are wholly interchangeable, but the audience for both is likely to be similar. Square-Enix seems to have considered this and, as a result, players subscribing to both games will get three bonuses for playing both games. The first is the most significant from a material standpoint -- the base cost of a subscription to Final Fantasy XI will be discounted to $7 a month for an account with both games linked.
The other two bonuses are for veterans of the game, but equally appealing. Players with a character already created in FFXI will be able to import their character name to Final Fantasy XIV, with the character name being the given name and the server name as the family name. (So "Jejeune" on the Fairy server would be named Jejeune Fairy.) And as a last carrot, FFXIV characters will receive a special item on creation, the Hermes Shoes. So if you're planning on picking up FFXIV and already play FFXI, now would be the time to link the PlayOnline account.