Shimano Dura-Ace Di2 RD-9070 Rear Derailleur

Shimano's latest electronic rear derailleur, the Dura-Ace 9070 seen here, represents the pinnacle of bicycle and electronic engineering. The biggest difference between this and the previous generation of electronic Dura-Ace is the switch to the E-Tube connectors that first made an appearance on Ultegra-level bits. This makes compatibility and programmability consistent across each of Shimano's electronic groups and allows a near-infinite level of customization. The derailleur you see here will auto-adjust itself, move itself out of the way in the event of a crash, and handle up to a 28-tooth cog. If you're putting together a dream build, Di2 is the icing on the cake.