The best person to answer this question is the person who will be using the sleeping bag, after considering for what it will mainly be used. A brilliant bag for extreme conditions has very different attributes to those of a lightweight travel bag. Warmth, weight, size and features are all important, but the order of importance depends on the end use.
Sleeping bags can be broken down into four main types, although categories do overlap:
Alpine bags are designed for extreme conditions above the snowline. They need to be both warm and light: a mummy shape, lightweight fabric and high quality down are used to give a high warmth-to-weight ratio, with water-resistant shell fabrics also common.
For bushwalking bags, the focus is on both performance and comfort. These bags are still light but have a few extra features and allowances for comfort, and may not use such high-specification materials. Bushwalking bags come in both mummy and tapered rectangular shapes.
There is less emphasis on technical features and warmth with travel bags, as they are generally used indoors. Packed size is still important, as is versatility and comfort, so most bags are tapered rectangular in shape.
General use bags are a little heavier, great for general camping, sleepovers, school camps and extra bedding at home, and are designed to be comfortable, functional and durable. Size, weight and technical features are not a priority, so most bags are rectangular or tapered rectangular in shape and robust in construction.