Which organism are you working with? 

I have had good experience with the ChopChop tool from Boutros et al. They have screened all exonic PAM sites for many species and given an arbitrary score. It is fast and all the guide's work in my hands (targetting 5 genes, 10 different guides).

The Zhang lab tool has much more customizability if you want to target intronic regions or use a double targetting strategy. Unfortunately you can screen only a relatively short portion of your gene of interest for CRISPR sequences (~250 bp). I have also designed guides based on their software and have not noticed any differences in efficiency with the Boutros application or a third software program.

So far I haven't seen any correlation with the arbitrary score of a guide and it's efficiency in vitro.

Best wishes,

Thank You Sir @Akker

@Pawat, HEK293T cells