It is mewtwo.
Arceus has the highest base stats - so? What you need to look at is stat distribution. Mewtwo has a base stat sum of 40 lower than arceus, but they are distributed in a logical sense - to make mewtwo strike first, and kill. Arceus's stats are evenly spread out, making him mediocre in everything.
And for every plate arceus can attach, mewtwo can use a super-effective move against him. AND because of the fact that mewtwo is alot faster than arceus, he will have the first move. AND, with mewtwo's outrageous special attack, combined with a supereffective move, will have arceus overwhelmed into a 1HKO.
Deoxys does not compare to either remotely... I'd put lugia right behind mewtwo and arceus.