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.