Correct 2 to 1 Multiplexer Truth Table
A multiplexer is a collection of gates where none are arranged to retain an internal state.  A truth table of all possible input combinations can be used to  describe such a device.  A 2:1 multiplexer has 3 inputs.  Therefore a complete truth table has 2^3 or 8 entries.  The truth tables in the question only has 4 entries and therefor falls short of describing a 2:1 multiplexer.  

Interestingly, most of the links in the question have 2:1 multiplexer truth tables that have 8 entries.

The switch diagrams are generally used in block diagrams where a 2:1 multiplexer is part of a larger circuit.  However, the more common symbol used look like this:



As for which input is passed through to the multiplexor's output based on the value of the selection line... If pressed I would say the 3rd table is the expected behavior.  I would expect a selection value of zero to pass the 1st input and a selection value of one to pass the 2nd and so on.  That is not to say the 1st table is wrong.  It too represents a multiplexer.  In fact, if all the selection lines were inverted (in this case there is only 1) this would be the expected behavior.