Cool tattoo designs mean its unique and special, not a lot have it already. So try to be unique and different yet don't go overboard as it might turn out to be an ugly one. Try to stay away from the fads just to be in as fads come and go. Try to think of a design that can last for a long time and still be in tune with time.
If you have the budget, have your tattoo artist draw a custom design for you. For a few extra dollars your tattoo artist might will be able to sketch out a cool tattoo of your very own, so you won't have to worry about having the same tattoo as anybody else in the world.
Always have meanings for your tattoo. Don't sport one just for its artistic beauty or the sheer beauty of it. There should be more to tattoo than just the physical look. What makes tattoo great are the stories behind them and you can always come out cool telling people about it.