What is the main difference between Emax veneers, CEREC, Vita Suprinity and Celtra Duo? I’m looking for most durable and natural

Hi Joanny,
I can completely understand that you're making a big decision and doing a lot of research into the options available. 
My recommendation is not to focus on the material being used, but more on choosing an experienced dentist who uses high magnification, and can plan your new veneers using photography and (even better) a computer simulation as a blueprint. They should be able to give you 'provisional' veneers which means you can test-drive the shape and of the new ceramic ones before you commit to the manufacture of the veneers by hand.
Bear in mind that if you want the most natural look, the laboratory technician your dentist chooses to work with matters just as much if not more! Feldspathic porcelain is layered by hand and the technician can create natural characterisation, but they aren't as durable in some circumstances. e.Max is very commonly used and is more durable, but can sometimes look more opaque. Then again, I've seen wizardry with e.Max from some ceramists! 
Cerec, Vita Suprinity and Celtra aren't suitable if you want the most durable and most natural.
Ultimately, excellent dentistry is never a quick fix, but it will be worth it. Like most things in life, you'll get what you pay for. Best wishes!
 

There are other factors besides material selection, although it is very important.  Tooth position, alignment, extent of overbite and presence of tooth wear and grinding are significant factors as well.  I would suggest having a diagnostic waxup or prototype model of your outcome which takes into account contours and how your teeth fit together when you bite and grind in order to minimize collisions and chipping long term.  This way you can also "test drive" a replica of the prototype in the mouth with temporary material to have more predictability prior to proceeding to the permanent restorations.  In terms of material, emax in my opinion has the best combination of strength, durability and esthetics for veneers.  But by having a knowledgeable clinician with a skilled laboratory technician and a good waxup, this could potentially allow you to have a weaker, but more esthetic material such as feldspathic porcelain.

These are all types of dental ceramics used by dentists and ceramists.I would focus on choosing the right dentist that works with an experienced ceramist to get the result you are looking for.  More important to ask to see before and after photos to get a sense of how your veneers can look.  In our office, we have a dedicated master ceramist that is experienced in layering porcelain to achieve a beautiful and natural result.  Equally important is having a vision for what the outcome will look like and knowing how to execute the veneer treatment from start to finish to yield a predictable result the fits well and is going to last a long time without problems.