There are two kinds of popular bags for Canon rebel T3i. One is backpack to hold Canon body, lenses, flash and other accessories. The other one is top loading bag to hold the camera with standard lens only, such as the kit lens.

The best Canon digital SLR backpack is Canon Deluxe backpack 200EG-black. This is the one I used over the last 5 years. It was built in high quality nylon and very durable. It can hold one or two digital SLRs, 4 lenses, external flash. There are many pockets to hold accessories such as memory card, filters etc. There is even a front webbing ideal for lashing light jacket, sweater, or rain coat. With the backpack, both hands and shoulder are free for taking photos. The only disadvantage is that it is quite hot when you carry the backpack on your back in a hot summer day. This is something you need to consider if you mainly use it in a hot summer day outside.

The best top loading bag is Lowepro TLZ20 camera bag. Lowepro TLZ20 is a water-resistant holster-style case made to carry an SLR camera body, lens and memory cards, batteries or small accessories.  The Soft, brushed-tricot lining protects the camera and LCD screens.  The main advantage of the top loading bag is that you can take the camera out quick.