For the quest-obsessed... Phantasy Star Universe (read our review)
Elder Scrolls IV: Oblivion is still the greatest pure RPG experience on 360 right now, but it's been out almost a year. Besides, Phantasy Star Universe is the one taking full advantage of Xbox Live's capabilities. Exploring worlds, tackling dungeons and, heck, role-playing itself is always more fulfilling with a friend. So imagine embarking on a planet-skipping sci-fi adventure with five friends. Picture each one contributing to the party in a unique way, such as mastering the sword or perfecting the art of magic.
Yeah, you can do that in PSU and, yeah, it's as fun and potentially crazy as it sounds. Best of all? The quest never has to end, as the game offers weekly content updates.
Runner-up: Marvel: Ultimate Alliance