I have a Dell v515w printer what is the best app to use so I can print from my iPad.

Is it a wifi printer?

Well, your Dell is not compatible with Airprint, the built-in printing app. You'll have to use a third party app or an app made by Dell.

All my printers are Airprint, so I don't use any printing apps. But, I've heard good things about Print Central, Print n Share, and Printopia. Check the app store for reviews.