Depends on how much you are willing to spend. What many people don't realise when they shop around for decals trying to find the best price is that the medium can vary widely.
By far the best to use is CAST film.. it wont shrink and it sticks down like a dream.. you shouldn't get any peeling away of edges with CAST.
Next down the line is Polymeric.. not as good as CAST but still a good performer
Last is Monomeric... often shrinks over time.
So if printing your own, then I suggest using a CAST if you can.. will get great results.
If printing for a commercial venture then most if not all suppliers will be using monomeric.. why?? Because people always buy cheaper.
No matter what you use, once printed, you MUST have it laminated.. can be Gloss, Satin or Matte.. it will protect from UV and scratches. It also make the finished product that bit thicker which means its easier to apply and tends not to stretch... many commercial decals are NOT laminated and believe it or not.. often they are not the cheapest!