The notion of best-case running time is kind of ambiguous for me. According to wikipedia, the definition of best case running time is:
The term best-case performance is used in computer science to describe the way of an algorithm behaves under optimal conditions. For example, the best case for a simple linear search on a list occurs when the desired element is the first element of the list.
According to this definition, the best case running time for BST insertion should be $O(1)$ [consider that we are inserting to the root node]. But different resources says different things, some claim that it is $O(\log n)$ [perfect balanced tree] and some others claim that it is $O(1)$ which one should I believe?