I use Benchling for gRNA design. It is very easy and efficient I would say. It calculates on-target and off-target scores based on logarithms Article Optimized sgRNA design to maximize activity and minimize off...
and these scores can guide you to choose the best gRNA and minimize off-target effects (https://benchling.com/tutorials/21/designing-and-analyzing-gRNAs). But of course it depends on the species you are working with and if that genome is available.
Thank Eleni Christakou for your answer. I have tried Benchling for a year now and I completely agree with you. The only problem is Benchling does not cover all the genomes which are available on NCBI at the moment. I used Benchling to design gRNA for more than 20 genes now, and I have to say that they all work pretty good.