The "best speed cube" differ among many speed cubers. There are generally two types of speed cubes, smooth turning cubes and fast turning cubes. Smooth turning cubes are cubes like the Guhong and Fangshi Shuang Ren and fast cubes are cubes like the Zanchi and the Alpha V.
I personally like smooth cubes and my main is the Guhong. Other smooth cubes are the Fangshi Shuang Ren and the Shengshou Aurora which I happen to really love and considering making it my main.
You can visit bestspeedcube.com
to see the several new speed cubes on the market and a comparison guide.
Hope that helped.