Unity is a multiplatform game development tool, designed from the start to ease creation. A fully integrated professional application, Unity just happens to contain the most powerful engine this side of a million dollars.
Honestly, the best engine I've worked with. It's amazingly simple but also amazingly powerful. It includes enough features in its free version to get you started on whatever project you might want to work on, and even get it to near completion before buying the pro license.
It understands my favorite programming language: C#. It uses my favorite framework: .NET. Chances are these are your favorite things too!
The editor is awesome, and it accepts tons of file formats for assets.
Add cross-platform compatibility to that, and you've got yourself a winner.