Rather than choosing between brands you should choose a camera for your needs and style of shooting.

TYPE OF CAMERA
You say you want professional pictures. That leads me to think you'd like to have more control over the types of photographs you take. In which case you'll want an SLR (single lens reflex). This will allow you complete creative control over exposure times for frozen action sequences or long star trails. It will enable you to control whether you have a pin-sharp landscape photo or a portrait with a beautifully blurred background that makes your model stand out.

MODEL OF CAMERA
If you want to take action shots, you need a camera model that can take many frames per second (fps). For landscapes you're not going to be that interested in a high fps but more the quality of the image, and if you're going to print it large you'll be looking at a full-frame sensor that enables you to capture more detail than a 'crop' sensor i.e. it has a bigger sensor size. Bigger sensor sizes also help with astrophotography where they produce less noise in low light.

For a beginner an entry level SLR of the brands you listed will give you the perfect grounding to improve your photography. Try the cameras out in the shop and work out which one feels right to you: are the controls laid out as you'd expect, that the weight suits you and that it has features you would find useful such as live view, video, flip-out screen, wireless flash control.

INVEST IN GLASS
Camera body technology changes rapidly. A wise investment is to buy good lenses. This will help produce sharper images with less lens flare, and when you're ready to trade up your camera body you can take your lenses with you.

THE BOTTOM LINE
It is the photographer that makes the picture, not the camera.

If you want some guidance read camera magazines and review websites to compare features and how the differing cameras and models cope with the types of photos you want to take.

its only a matter of what you prefer though. i asked some of my friends and they mostly will go for canon. so i pick canon because i can go to my friends if i had problem. and i am very satisfied :)

A bit like asking 'which is better; Windows, OSX or Linux?' The answer is down to a mix of personal preference and specific need. By this I mean different people will have different views. I'm a Nikon man myself for stills, Sony for video. Had Canon and they are very good too. Sony apparently do good pro still now and since they took over Konika Minolta have further added to their pedigree. I believe the top end Sony cameras have a particular reputation for image sharpness. How to decide? Ask others (you are). Try handling the cameras you are thinking of yourself. Read reviews (mags and sites like dpreview.com and steves-digicams.com ). Decide what kind of photography you are most likely to do (this impacts the features you need and what aspects of handling should be easiest). You should be getting close to an answer about now but as you can see quite a bit of groundwork is required!

sony is great for point and shoot and takes fairly decent pictures, canon is great for taking pictures of things that won't stay very still (think children) and nikon is the best for taking pictures of landscapes and macros of small things. Nikon has also been in the camera business the longest over the three choices, so that is who I would go with. Also, consider optical zoom first over most any other option when it comes to digital cameras. If you are spending anything more than 499 that you should get at least a 10x optical zoom.