The venerable Shimano Altus rear derailleur. Solid and affordable. Technically 9spd.

• Cage: SGS (long)
• Total capacity: 45 teeth
• Max front difference: 22 teeth
• Includes cable adjuster
• Conventional direct attachment
• Max low gear sprocket: 34-teeth (unofficially this works with 36t if you're using a double crank)