You shouldn't use getglobal(), as it's not only deprecated but is now actually a function-wrapper around _G and is thus actually slower due to the overhead of the function call.
The rest are functionally equivalent, the main difference being syntactic sugar. The only time I use _G["whatever"] is when I am passing in an unknown string (as through a function parameter) or constructing a string; otherwise I use _G.whatever.
Edit: Damn it, Dridzt!
