Sirfetch’d's current moveset is:
Leaf Blade - Almost always results with a crit because of its Leek
Close Combat - STAB
Brave Bird - It's a strong move
Iron Defense or Swords Dance? I’m using this moveset for online battles in Pokemon Sword.
Swords Dance beyond a shadow of a doubt. Sirfetch'd has an insanely high Attack stat, and its Defense is only mediocre. Iron Defense is better suited to defensive tanks like Avalugg, not attackers like sirfetch'd. The duck wants to hit as hard as it possibly can, so Swords Dance should be the clear option. Iron Defense is practically a waste of a moveslot.
Hope I helped!
Definitely Swords Dance. You're more likely to take less damage if you OHKO your opponent after a SD rather than taking an extra hit to go for Iron Defense.