I have a project to invest fake money in the stock market;what's the best stock to buy & get a big RETURN?

I suggest you look into Headwaters (HW) or Aspreva (ASPV) both of which have the potential to significantly appreciate in a short time frame, while not being so incredibly volatile that there is a large chance you lose alot of your money.
Yes, alot of the strategy depends on the rules, but the general idea is to take on above average risk. Don't go with the MSFT/GE/BAC mega-caps...
For more reading on the two stocks I mentioned, you can find free reports on them at http://www.valuestockreports.com/stockre...
Good luck.

what is the game? I have been trying to find this and cant i have heard a lot about it

The biggest potential returns come from the riskiest investments. Technology and biotechs are good possibilities for big returns, but they could go down just as easily.

Buy Google stocks it always goes up. It went down this week by 30 points because people got nervous and started selling their stocks but it's going to go back up when people gain back confidence in the market. Google is the most active stock and they said it would go up from 400- 600 a month ago. It's currently at 438.

If you are paper trading- go to smartmoney.com- set up a paper portfolio- and you'll be able to track every detail of your portfolio as if it was a real one.
as far as big returns - you have to consider the risk/reward tradeoff and your tolerance for highly risky stocks...
most risk is associated with volatile industries- right now these include airlines, biotech, and energy.
When you set your portfolio up- remember you don't want to put all your eggs into one basket- you definately want to diversify- so choose a variety of stocks- including those that will give you solid returns. For that, look into GE, Berkshire Hathaway, and any defense contractor winners (ex Boeing - which has had a 3yr return of 150%)
good luck!

If the projects goal is to receive the biggest return possible and there's no downside to losing then I would suggest simply picking any volatile small cap stock with an earnings date coming up and put all of the money in that one stock. Be aware that It's not an investing strategy but rather gambling and that's why these types of projects that reward you based on absolute returns without discounting for the risk assumed are absolutely worthless as far as an excercise in learning about the real world of investing.