Which is the best tool for mobile UI designing sketch or illustrator.
There is more than 1 right program for the job. If Photoshop works for you, then use it. Is there something you think you're missing by using Photoshop and not something else? A lot of people do like Sketch because of how well it works with for creating UI Designs and exporting assets. The biggest upside to using Sketch or Illustrator would be creating images as SVG.
There are some other factors of do you work in a collaborate environment where someone might need to work on a working file? This could be a problem if you start using Sketch.
I think someone commented on my answer (I don't see it right now?) saying that you can export SVG from Photoshop. I guess this is partly true. Someone posted a link to a free script Free photoshop script: Export your vector layers from PS to SVG. The downside to using something like this is that all of the objects have to be merged onto a single layer. The functionality of using Photoshop for creating SVGs in my opinion is not worth it. You can also export the paths to illustrator and save from there.