There's only one objective answer to give: 10.
You don't sell a game for billions of dollars if you didn't do something right.
Markus took a great concept and kept adding content to it, engaging with the fans, and building a community that ended up taking a cult game to being a global powerhouse. While there were definitely missteps, by and large Notch and the people around him kept making the right choices to build this well-developed little idea into something that one can sink thousands of hours into.
I'm not sure if in the long run Minecraft will be viewed as an oddity, as merely the fetal beginning of more creativity-driven user-created experiences, or as a watershed moment for gaming, but either way, for the foreseeable future, Minecraft will cast a simply inescapable shadow over the gaming industry. No matter what occurs, Minecraft will be an influential text in the gaming lexicon, informing thousands of games to come.