I have some opposing opinions from what Flaw said, so I'll just give it here as a separate answer and let the voters decide if it's justified. There's hopefully some truth in both answers.
First, it is my understanding that ～ろ is simply not used unless you essentially want to bark at someone.
～なさい would be used not when you're "socially higher", but when you want to put yourself higher than the listener because you think you know better. Thus it would be quite impolite to use when talking to other adults, but it's common for commanding children.
～て is informal as Flaw says, and ～てください essentially becomes "please do ..". Hence the first is ok among friends or when you're talking to someone socially lower than you, and the second is more appropriate when talking to strangers or aquaintances (note that they don't have to be socially higher than you, but if they are, the second is more appropriate).
I'm not sure about ～てくれ more than that it comes from くれる as Flaw points out. Here I'd just like to say that requests are often better made in the form of a question. (would you (please) do .. (for me)?) One way to do so would be ~てくれませんか, although this one is best used among somewhat socially equal parts. If you're talking to your boss or some such, it would be more appropriate to use ~ていただけませんか. I'm sure there's even more polite ways to make requests, but none that I am aware of at this point. And if you want to ask a close friend to do something for you, something like ~てくれない？ may be the way to go.
I hope I gave accurate descriptions, I think it's really difficult to think about these things as "this one is used in this situation". It's probably better to understand the "feel" of the different words/phrases (there's a word for that, I'll edit as soon as I find it.. connotation?) and go from there. Then of course the one thing holds true no matter how you form your request/command is that informal speach patterns is best used among friends or socially lower aquaintances. For example, a waiter may be socially lower than you, but it's probably still appropriate to ask for help with the cooking at the table using ~てくれませんか.