Most of the time I would answer with, "I like Multiplayer content better" because I've played MMOs, Online Games, and most of my friends are online. People say these people are fake; turn-offable friends. But I'd never just leave them and ragequit, and these games are fun because of these people you want to do better with, to beat the content set before you. Whether this is first-person shooting or an expansive MMORPG, content is always less drab and boring when you're playing with other people and living your own story even through a content-guided path - nobody's journey will be the same, no two people will be identical. It's fun to just slip into any assumed identity online and be a certified badass.
Now, I like Single Player too. I like Campaigns and Stories as much as the next guy; but after a while no matter how inviting they are, singe-player games have me going back to multiplayer because I get bored.