The Daily FreePlay tournaments will have the largest pool of opponents since it is free to enter and is available to upgraded/cash members*.
Don't beat yourself up if you rarely win/place within the Daily FreePlay tournament as the winners of the Daily FreePlay are considered to be the best on the site (and even in the world!).
*Daily FreePlay tournaments are only available to "Legacy" accounts. "Legacy" accounts are defined as cash accounts that were upgraded prior to 8/1/17.