Really, I've found that its rare for a game to be good at both, I like both equally.
Single Player games like Half-Life and Mass Effect are awesome, but games like Battlefield 3 are multiplayer only in my book, the single player was so bad, it hurts me knowing they actually spent time doing it.
But yeah, I like both Single Player and Multiplayer games. I think Single player edges out slightly ahead though as it is more of a personal experience and usually if a game is telling a story, a single player experience is better to convey the atmosphere, only games that change this rule is Left for Dead and MMO's.