Some might say NSMB Wii due to multiplayer, but between playing NSMB Wii solo and Mario Galaxy, which one is more fun in your opinion?

NSMBWii. It is a blast to play, even on your second, third or thirtieth play-through. Can't say that about Galaxy.

Mario Galaxy is one of the best, most wonderful and beautiful games ever and the best of this gen.

NMBwii is the worst mario game in a good while.

NSMBW - busted a gut laughing with friends playing it.

and frankly, SMG bored me.

I want to stress that both are stellar games, but after completing NSMBWii, I quickly lost interest.

Mario Galaxy on the other hand... I've never seen such an amount of creativity and love and diversity poured into one game, I took my time and I was smiling from start to finish. Best game I've ever played, I think, and I can't wait for the second part.

NSMB Wii.

SMG was the better game though. I can't really explain that.

NSMB Wii easily. It is not even a contest.

I'm like most of you:
SMG was a far better and more polished game...
but I had more fun with nsmbwii for unknown reasons

Ignoring the fun to be had with friends, then Galaxy but the reality is that NSMBWii has multiplayer that it does while Galaxy can not compare.

New Super Mario Brothers Wii is way more fun. I don't remember playing Galaxy much after the first week or two I had it, but I have been playing lots of New Super Mario Brothers since it came out, and to this date. According to Nintendo Channel, I have played it three times as long already (though I am not the only one who plays that one).

Mario Galaxy was very well done (much better than Twilight Princess), but the pacing was too slow. Slow paced games should use story a lot more to make them more rewarding. Otherwise it was very well polished, but the plotline and adventure elements needed more work. This is why New Super Mario Brothers feels like a better game, because it did everything right for the 2D sidescroller genre in order to make it rewarding.

Anyway, New Super Mario Brothers was the first Mario game to do Mario Brothers right since Mario World. It was the true sequel to the main series, after 18 years.