Codex Mechanicus by a long shot.
this army could still be beaten with space wolves. hit matt ward with x4 jaws and he just might go down, provided you get to go first. if you go second then....well...Codex: Matt Ward.
It contains just one unit - Matt Ward
cost - free
WS 10 BS 10 S 10 T 10 W 10 I 10 A 10* Ld 10** Sv ?***
*Ward gains a number of attacks equal to the sales of GWS that year in addition to his base 10 attacks
**Matt Ward never takes leadership, pinning or morale checks as he is unquestionable
***Ward has a 1++ Wardsave against all and any attacks
Wargear - fanboy mind: Ward is a psyker that causes all enemy units on the board to suffer a Ld check at Ld1. Any unit that fails is removed from play as it cannot grasp Ward's pure awesomeness.
The Mangler - this is Ward's personal fluff-fantasy book that counts as a power fist that strikes at normal initiative in combat due to its weight and retcon power.
Wardpistol - this is a S10 AP1 Assault 100 weapon with unlimited range that fires his awesome ideas into the minds of his enemies, liquifying their brains
Matt Ward - As Matt Ward is Matt Ward and therefore no one can contest his awesomeness or godliness, and army with Matt Ward on the battlefield wins the game automatically on a D6 roll of 1+
Seconded.Codex:space skaven (imagine that a huge incursion of these guys is happening now, and it turns out they are the mortal enemy of the tyranids and came from their galaxy. They have been mostly eradicated from the universe and have come to our galaxy to try to stop the tyranids from destroying this galaxy too. They are extremely sympathetic and want to help, but are unprepared for the harsh and xenophobic races of our galaxy).