Which camera is best to buy? Sony Alph SLT-A37, Canon 550D & Nikon D5100?

Camera suggestion…

The Sony has a slight edge in video, but the only way to choose is to handle them and find out for yourself.

I'd add the Pentax K-r to the list as well.

I'd go for the Nikon myself. But that's only because I'm a Nikon guy, and have been using them since 1986.
My suggestion, go to a store that sells all three of them. Each one will have a different feel to it. They do to me. Each of us has different size hands, and a camera that feels right to me, may not feel right to you.
You can also go to a site like amazon.com and see what other buyers are saying about each camera.

Have you done enough research to know what features each camera has?

Did you know, for instance, that the Nikon D3200 has not only a more advanced sensor than either of those listed cameras for a lower cost?

I suggest you take a look on this website to see how each of those sensors perform

http://www.dxomark.com/...nd3)/Canon

NOTE: The SLT-A37 has not yet been tested.