If the sprocket is off the bike you can't beat Gunk or Jizer. They're too strong though to use when in situ around the chain. Not good for the grease in the pins.
Muc-off or Wurth is good for general cleaning. work it a little with an old toothbrush if it's stubborn.
When you refit the sprocket, nip everything down in a star pattern, then torque up in the same way.
Have fun
