For Farfetch'd, Slash is definitely better because his ATK is higher than his Sp.A... But, he still should have a Flying-type attack. Regrettably, Farfetch'd doesn't have much to work with though. He can learn Air Cutter, Air Slash, Aerial Ace, or Pluck. Pluck is more powerful than Air Cutter (but not by much) and has a neat effect, but Air Cutter can critically hit.
Other good attacks Farfetch'd can learn are Night Slash, Steel Wing, Leaf Blade, Double-Edge, U-Turn, and Return. I'd suggest you use something like Knock Off or Toxic, though, since Farfetch'd isn't too strong himself. Farfetch'd also learns Poison Jab, but it gives him poor coverage.
If you can get a Stick hold-item, I highly suggest you use
Farfetch'd @ Stick
-- Night Slash
-- Slash
-- Air Cutter
-- Leaf Blade
Otherwise, use stronger variants of the attack (Return / Double-Edge over Slash, and Air Slash / Aerial Ace / Pluck over Air Cutter). Though, to be honest, Farfetch'd isn't going to do much without critical hits anyway.
