Which are the most effective sorting algorithms for a large dataset?
quick sort. in C++, it's better to use std::sort rather than qsort(), since it allows compiler to perform much better optimization (both implements quick sort algorithm)
depending on your data, sometimes better: http://en.wikipedia.org/wiki/Counting_sort and http://en.wikipedia.org/wiki/Radix_sort



  simultaneously perform sorts on multiple large datasets?


i've not heard of such algorithms at all. but you can distribute tasks among CPU cores. moreover, you can split one dataset between two cores, and then merge results of sorting