5.3, because it works on damn near everything [like say, VirtualBox w/o hardware acceleration]. GM8 is really nice, but I'm almost never in Windows natively, so I can't really use it.
Shame, because I love the proper alpha support in GM8, and rotation is a godsend [hell, it has been since GM6, but it sitll deserves mention].
Also, I was one of the people who backported to GM5, using not-Windows meant GM6-and-up stuff broke [eg, VirtualBox without hardware acceleration, which I didn't have at the time], or ran really slowly [Wine].
I haven't actually seen if GM8 works well in VirtualBox, if it does, I might switch back.
But... I haven't actually worked on a game in ages though.
Also, I'll be amazed if I see anyone making GM4 stuff, unless you're targeting Windows 95.