Fireworks is great, but whether it's the best depends on what you're actually trying to achieve at the time. Photoshop and Illustrator are both more appropriate on occasion.
I do love Fireworks, and it's an essential part of my toolkit, but if I had to only use one program, then I'd pick Photoshop. I'd miss out on the superior vector-based graphics in Fireworks and Illustrator, but I wouldn't be able to do without the control for custom filters and the industry standard file formats that my clients and associates require their images saved to.
In the real world, I use Photoshop, Illustrator and Fireworks and wouldn't want to be without any of them, as they all have their strengths!