Im not really sure, I'd have to say codeblocks even still, because you can use a cross platform wxWidgets toolkit for GUI stuff which does allot more than .NET framework controls. It can do MDI and allow docking floating panels, it also has a Scintilla wrapper, which is like a code editor with the line numbers and stuff and it can be customized and parses pretty much all of the known languages, being XML HTML C# C++ Java Basic you name it. But if you really just want an easy to use game programming IDE and language for only Windows development, use XNA & Visual C#.
I've said this before and I'll say it again.
When somebody is starting out, learning things for the first time, ambitions of being cross-platform should be the last thing on their minds, the bottom of any concievable priority list.
They have enough to be doing as it is. There's a huge body of knowledge that they need to build up on just one platform; multiply that out by the number of possible platforms they may wish to hit and you've a recipe for failure. No, libraries and toolkits aren't good enough - there are still enough platform-specific quirks that they fail to abstract out fully. Learning new stuff, making it work, dealing with mistakes and misunderstandings, and having to do it all over again for each aditional platform - that's just not gonna fly.
There's nothing wrong with aiming to go cross-platform, but just not for the first project. Recommendations to (paraphrasing here) "use OpenGL because it will help you get on the iStore" are neither helpful nor constructive here, nor are they even remotely on-topic with respect to the question actually being asked.