I use the Canon system.

My main camera is a 20D. My favorite lenses are the Canon 10-22 f/3.5-4.5, Canon 17-40 f/4.0L, Canon 28-135/f3.5-5.6, Canon 80-200 f/2.8L (an older but great lens), and Canon 100-400 f/4.5-5,6L . A tripod is very important for nature photography. I need a steady platform for all exposures, especially for the longer ones. For that I have a Bogan 3221 with Kirk BH-1 ball head.

Why did I pick Canon? When I wanted to upgrade from my Canon AE-1, I held a Nikon, a Pentax and a Canon in my hands. The Canon seemed to fit my hand better. The controls were placed where they worked best for ME. Quality was very excellent on all three brands. So, my deciding factor was comfort.

All of the major manufacturers make excellent equipment. If you are looking to get into the SLR world, decide which "system" you want to go with. Lenses and other accessories typically fit only that brand of camera. Once you have an investment in a system, it can be expensive to switch.