I'm searching in several doc but I'm not able to understand what is the recommended environment for Microsoft SQL server 2012 / 2014, that is guest OS and ESXi version.
Actually I have ESXi 6.0 U3 .
What is your experience saying ?
I really don't understand what are you asking for... the recommended environment, do you mean only the Guest OS? It can be whatever you want from Windows Server (just be aware of the EOS versions) and regarding the ESXi the same. There isn't a "recommended" environment, the one you can support, the latest should be the best but also can have some bugs.
So, as I said there isn't a recommended environment based on the version of the Hypervisor (in this case ESXi 6.0U3), if you can get one that is supported instead than a legacy one, better for you
It's not a single line answer, you have to plan it with workload, availability, performance and resource perspective. Someone would barely share their experience here over this topic because there's so much to do deploying and operating SQL, you have to be specific with your question. Please refer to below best practices guide for SQL on VMware.