Guys, Contrary and V-Create just might allow Farfetch'd to kill Scizor...in the sun. Maybe.
The problem with Contrary is that you will be hard-pressed to get a move off in the first place. Farfetch'd is not only weak to Stealth Rock, but it has the defenses of a wet paper bag. Good luck switching into any attacking move. If you manage to switch in, you're still extremely slow (60 Base Speed is just a little better than Blissey), and will likely OHKO'd by anything with even average attacking stats. If you make it THAT far, your first V-Create/Overheat/CC/Leaf Storm will be incredibly weak...even the high base power doesn't save Farfetch'd from the fact that it has a 65 base Attack and 58 Special Attack, and those attacks are Un-Stabbed to boot.
If you miraculously manage to fire off a single attack and get that boost...so what? You're at +2 Defenses and Speed, but you still lack the offensive power to scare off anything. Close Combat is the same, but now you are without your speed boost. Leaf Storm/Overheat have the opposite problem. Sure, you can now actually threaten things with your attacks, but you still cannot take a hit nor outrun anything worth mentioning. If there was a move that decreased all stats by -3, then you might have a case for giving Farfetch'd Contrary. As it stands however, it doesn't do anything for it.
tl;dr: Contrary + anything will make it good enough to get an honorable mention on a sub-NU threat list at best.
Now, onto the actual question. We have to look no further than Ditto for one of our best answers - give Farfetch'd Eccentric. This fixes almost all of its problems in one full swoop - those being hideous stats, an awful movepool, and a terrible typing both offensively and defensively. Although it might not make it good enough to get to Uber (time will tell how good Ditto is with it) I think most would agree that it would at the very. least become viable in the OU metagame. For something that is the laughing stock of Pokemon, that is a huge jump. It wouldn't even need a move with Eccentric.
Of course, another option would be to give it Inconsistent. Granted, Inconsistent wouldn't make it Uber in the sense that it would be over-powering anything, but if you took away the options of its other abilities, Farfetch'd would by definition have to be banned! (Or the Inconsistent ban would have to be lifted, but I don't think Farfetch'd would cause it to be repealed, especially since Bidoof has it) Even if it that didn't happen, Inconsistent Farfetch'd would still be banned from competitive play.
An honorable mention is to give Farfetch'd Drizzle + Hydro Pump. As we saw with Politoed, Drizzle is clearly one of the better abilities in the game, taking a forgettable Pokemon into a Top 5 OU Pokemon. Farfetch'd wouldn't make near as impressive as a jump as Politoed did due to its poor stats and inability to abuse the ability itself, it could still provide support for Rain abusers in the lower tiers. Though Drizzle Farfetch'd wouldn't be Uber, it could at the very least find some usage in UU/RU. (Think Snover, Hippopotas, and Vulpix)