Aerial Ace's power is 120 when super effect.
Return's power, on a Slaking, is 153.
The only time Aerial Ace would be better is vs Heracross or Breloom,'Loom isn't too common and Slaking should flee from Hera anyway(you wanna keep that monkey healthy).
Focus Punch helps it deal with Skarm and Fury Swipes breaks lotsa subs and deals damage.
So,I repeat:
"Slaking@Choice Band
Adamant
252 Attack/252 Speed/6 HP
Return/Double Edge
Earthquake
Shadow Ball
Fury Swipes(breaks most subs)/Focus Punch(for tricky pokemon that resist the other 3 moves)"
299 speed beats flygons and wacky heras and stuff,so I prefer it over 270 =D
Edit:Ha I won! XD