Depends on the game.
A racer would generally lead me to say multiplayer, although there are exceptions for that too, my most recent purchase being F1 2009 for Wii, which has no online component... and I am glad of it, because I can be pretty sure playing against real people would be a horrible experience for me playing Formula 1.
If Nintendo make an F-Zero for the Wii though, and don't include some sort of online then I will cry (30 player online would be awesome, but I would be satisfied if they can't manage that many and take it down to 12 player like MKWii)
Obviously multiplayer adventure games rarely work as well as their single player versions, while FPS are much more a multiplayer thing, often with a lacklustre single player tacked on.
======
Overall I will say single player, and my personal game time more than backs this up (I probably spent 90%+ of my gaming time on single player aspects of games) but I still love multiplayer.
Essay Pro