On December 21, 2015, Microsoft released SQL Server 2014 Service Pack 1 Cumulative Update 4, which is Build 12.0.4436.0. This CU has 34 hotfixes in the public fix list, which is still a fairly substantial number of fixes. This shows that Microsoft has been busy fixing quite a few issues to further improve SQL Server 2014.
If you are still on the SQL Server RTM branch (which I think is a bad thing), Microsoft also released SQL Server 2014 RTM Cumulative Update 11, which is Build 12.0.2560, on the same day.
The official Microsoft build list is available from this KB article:
This KB article has a lot of very useful information on how to properly configure SQL Server 2012/2014 for the best performance and scalability: