Every mountain biker should have a bike light in their pack, attached to their helmet or bike handlebars. The main question is how much are you willing to spend on a good bike light. I have tried these 3 out and I would put them in this order: CECO F1000 ($40) is number three.
The SIGMA Buster 700 ($40) is number two. Even though it lacks some lumens compared to the CECO I feel like it has a more heavy duty construction and is made to last. In comparison the light it put out was equal to the CECO.
The Lumina Dual 1800 ($165) was number one. The 1800 lumens drastically outshined the other two and it appears to have a solid, well constructed build, but you do have to pay for it.