Nikon D3200 SLR vs Canon EOS 600D SLR vs Sony SLT-A58K SLR: which one is best to buy?

Which is best one?

The best one is the one that you feel the most comfortable with. All three will take great photos.

There's no way to tell really, because it really just comes down to personal preference. I personally am a big fan of canon, although I know a lot of people who like nikon and sony. I think you should go down to a store and get some hands on experience.

The quality of each of those cameras is excellent. You can't go wrong with any one of them. What is important is the feel of each of them in your hands. Take yourself to a camera shop and try all three of them. That's when you will know which one is for you.

D3200 because of much better low light performance.
Sony if good video control is very important to you.

I'm a canon fan so would recommend the 600D
Nikon fans will recommend the D3200 and Sony fans the SLT.
Ultimately it's your choice.

Probably the Sony A58. I still have my Sony A65, and a Pentax K-5 (The K-5 is far better). I had a 3000 series Nikon, and the colors are fairly bland, and the Canon's are more-so. The A58 is still sharper than the other two because its probably running anti-moire software, so the anti-aliasing filter is lighter, and hence why the images are sharper.

Imaging resources comparometer (click on a camera, and open up "still life ISO 100" then click on the image to make it full sized. Then compare against the others)

http://www.imaging-resource.com/IMCOMP/COMPS01.HTM

Even the A65's looking better at ISO 6400 than the D3200. Look, the A65 is a mid range camera, and is better then the other tow.

But get an idea of the images here:
Sony A58

Nikon D3200

Canon 600D

The Sony will give better pictures that are sharper with truer colors, and with zero mirror slap you get with a conventional SLR. The features will be better. The auto focus will be faster too. The Sony's "Steady shot" sensor stabilization is better too at 3.5-4.5 stops, plus you can use wide angle and prime lenses stabilized. Sony also has focus peaking for video manual focus. Its just too good! The other two can't compete.