1)**Merge Sort** is unarguably the fastest sorting algorithm with a worst-case complexity= **O(n logn)** . This means that whatever be the arrangement of the elements in the array, it won't take more than n*logn steps to sort the array. A standard computer generally takes 1 second to perform 10^8 such ~~steps/oprations. ~~steps/operations. An important point to note here is that the base of the logarithm is 2 and not 10 or e as generally taken in schools. The **only disadvantage** of Merge Sort is that it requires **extra space** , i.e. an extra array to merge the sorted elements into. This is important to know because sometimes additional space has a great cost attached with it.
2)**Quicksort** is usually the sorting algorithm that is used everywhere. A few examples of this would be the inbuilt java, C++ sorting functions, MS Excel ,etc. Quicksort has an average worst-case complexity of **O(n logn)** but a worst-case complexity of **O(n^2)**. However, this only occurs when the array is already sorted and can be overcome by randomizing the position of the pivot everytime. This is a very convenient technique since it does not have the additional cost of an entire new array.