I'm plannning to photograph negative film like this. And I would like to know what would be the sweet spot of my lens to achieve best sharpness possible, what f stop and at what mm I should be shooting, also I will be using extension tubes. Thank you.
35mm to 55mm at f/5.6 to f/8.0 will get you great results with the EF-S 18-55mm STM.
I really like SLR Gear's visual interactive graphs for checking out lens image quality at various focal lengths and aperture settings. SLR Gear Lab Test Results
This is what a GREAT lens looks like, and below is what a CRAP lens looks like:
Note the minimum focal distance of the 18-55mm STM is 0,25m so this is part of the limitation on what you can shoot. Photozone indicates the maximum magnification is at 55mm, which is probably what you should use regardless of "sweet spot". Distortion, CA and vignetting are best at f8 and above and resolution at f8 is as good as you'll get across the frame. So 55mm at f8.
Strictly speaking you should probably use a macro lens for this if possible.
This is not an answer to your question, but a suggestion for your setup. In order to help minimize any extra glare or reflections off of the front of your negatives or slides, try to block out the rest of the light table.
Or, create some "gobos" (black absorption panels) out of dark construction paper, or black matte-painted foam board, and use them to surround the field of view to create a dark tunnel where your slides are exposed.