Unless specifically modified, the default trigger method for all stocks is the "Last" method as described within the User's Guide.  

For US options the default trigger method is the "Double bid/ask" method, where two consecutive ask price (bid price) values must be less than (greater than) or equal to the trigger price, and the second bid or ask must have greater size if it is at the same price level as the first bid or ask.

For Forex cash pairs (for example EUR/USD currency pair), the default trigger method is single "bid/ask", where the ask price (bid price) value must be less than (greater than) or equal to the trigger price.  

For all other contracts like Futures and Futures Options contracts the default trigger method is the "Last" method as described within the User's Guide.

For additional information see: How to Modify the Stop Trigger Method.