The fastest user of Me First is accelgor, though when male meowstic with prankster uses it it's faster due to prankster giving plus one priority to status moves, but in terms of base speed it's accelgor.
Top ten fastest users of me first
2.Mega Mewtwo Y
3.Mega Mewtwo X
6.Mega Absol and Zebstrika when evolved from blitzle (same base speed)
8.Mega Pinsir and Mienshao, Mismagias when evolved from misdreveas, Floatzel ( same base speed)
If including Prankster, Meowstic is faster