Shimano

XTR Di2 SC-M9050 Digital Display Unit

Figuring out the perfect gearing requires some fairly complex mental math or educated guesswork to identify which combination of cogs works best, but Shimano's XTR Di2 electronic drivetrain does the work for you, and the SC-M905 Digital Display Unit feeds you all the relevant info so you know right where you are, drivetrain-wise. The secret is Shimano's Synchronized Shift, which automatically chooses the right chainring and cassette settings every time you shift, so you'll always have the perfect gear ratio with only a single shift lever to worry about. Before it changes things up on you, the unit will emit an alert tone, so you don't spin your legs unexpectedly or find yourself stuck in the mud.

Of course, if you want to choose your own ratios, the SC-M905 also has a manual mode that lets you shift more traditionally while maintaining the unbeatable crispness of Di2 electronic shifting. It can even adjust your derailleur on the go, if it gets knocked out of whack, relay the climb/descend settings on Fox forks and shocks, and tell you how much battery life your drivetrain has left, so you don't find yourself stuck in the wrong gear at the base of a killer climb.