There are many good sidekicks out there, from Robin to R2D2, the best is rather difficult to describe. There have to be objective criteria to fallow in order to determine this question.
A true sidekick operates in compliment to there leader counterpart. This would rule out someone who breaks the moral code of their leader. However they also have to hold their counterpart accountable. In this fact Robin or R2 fit the bill.
Furthermore a sidekick has to be able to operate without their counterpart, that is to say the sidekick has to be able to be a hero in the absence, incapacitation, or capture of their counterpart. This would rule out the sidekicks like Diggle of Arrow as he regularly walks away if the Arrow himself is not present.
Given these two simple criterion I have to go with Robin as the best sidekick, Not a specific Robin, but rather the very idea of Robin. Furthermore the Robin mantle passes from one person to the next with each one graduating in a sense to be their own Hero, while still being a part of the Bat family and coming back to assist Batman as needed. The Robin mantle is rarely unfilled for long and whoever is filling it has a knack for grounding batman.