I'd have to say that Game Maker is probably the best 2D game development IDE out there.
I disagree - I'd say
Construct takes that cake at ease, even though it's still not in version 1.0 and might be a bit unstable at times. The runtime is notably more efficient than Game Maker's, it's fully hardware-accelerated from bottom to top, the event system is fantastic, and can be extended to pretty much anything with Python (for example, I recently integrated SQLite into my Construct project using Python, and now my inventory works better and simpler than ever). It also has some limited 3D capabilities, but it's main purpose is 2D game development.
I would seriously suggest people to give it a spin, even if they've been using MMF/GM for years. I personally moved from MMF/MMF2 to Construct, and never looked back.
Another reason to give it a try is that it's completely free as well (and open source to boot)!