Shimano XTR M970 Rear Derailleur Low Normal (Long Cage)

There's a good reason why you'll find the XTR Rear Derailleur aboard just about every top-end off-roader. It's super light, durable and shifts like a dream. This cold-forged-aluminum rear derailleur is stiffer than ever and boasts smooth, light-action shifting. The 11-tooth pulleys alternate link contact, reducing wear and smoothing pedaling. Plus, the Low Normal shifting action is super smooth.
Chain-wrap capacity: up to 43 teeth
Largest cog this derailleur can handle: 34 tooth
Weight: 205 grams
Low Normal means that the derailleur spring assists shifts to the larger cassette cogs, which speeds and smoothes shifts and greatly reduces the chances of damaging the derailleur during upshifts because you can't force it with the shift levers.