Ian, I disagree that Wii Play can't hold claim to the record. Just because you get a free Wii Remote with the game doesn't change anything.
I think it changes EVERYTHING. I'm pretty sure every single person on this forum that owns Wii Play bought it because they wanted another controller and the price difference between a standalone and Wii Play was so minor that they figured they might as well get the controller that came with a game. Every person I know personally who owns Wii Play bought it for that reason. That's like regarded as the thing to do. Salesmen in stores point you in that direction when you buy a Wii.
You don't get a free remote with it. It's the remote pack-in title. They just offer standalone remotes as well because someone who wants four controllers wouldn't have need for three copies of Wii Fit. If Nintendo didn't plan it that way they fluked their way into one of the most successful strategies of all time.
I look at Amazon.com's user reviews and I get:
"this is a 10$ game bundled with a remote. Only worth buying because it comes with remote"
"The whole package was the same price as just the remote from Wlmart. So it was worth it. The games arent all that much fun...just ok"
"I don't believe my grandchildren play the games that came with this remote."
"I bought this really because it had the remote with it, but the Wii Play game is actually not bad."
"We bought if mainly for the remote and count the games as an extra benefit."
"I bought this game because it came with a free controller. I played each of the mini games once. They are beyond bad. My kids played once and never put the game into the Wii again. Good for controller, but I could have saved a few buck by just buying an extra controller by itself, because the game is worthless."
"The main reason I bought this game was because it was a bargain with the Wii remote."
These don't appear to be written by disgruntled core gamers. There's no "rrrgh Wii Play sucks" here. This is the mainstream Wii audience and the message is loud and clear: you buy Wii Play for the remote!
If Wii Fit is number three behind Wii Sports and Wii Play then it is the best selling game of all time and its record is completely legitimate.