Chewie was unquestionably the best side kick ever.
What didn't Chewie do for Han? He was an excellent pilot, one who even Han would give the Falcon to while he went to the gun pod.
He's a great mechanic - he helped Han keep that ship running for years.
He helps Han find new business, which is shown in A New Hope when Obi-Wan first talks to Chewie who brings them to Han.
He's smart - as a recreational hobby he plays holochess. And seems to be playing well AGAINST A DROID. It's pretty hard to beat a computer on hard mode, think of what it takes to play against R2.
He's normally the one who gets Han to listen to reason. You can see his disappointment that Han is taken in by the "she's rich" ploy, but he follows the plan just the same.
Chewie is an excellent shot. During the confusion in the detention area you can see him blasting away at target after target.
He's the conscious - the Jiminy Cricket - to the scoundrel that is Han. What we never get to see in A New Hope is Chewie getting Han to go help with the attack on the Death Star.
He's the perfect foil to Han, keeping him grounded even when Han is busy flirting.
Hes loyal - nobody can hear his utterings when the blast door closes on Hoth and not feel something.
How loyal? Shall we remember he likely figures he will certainly die once he starts flinging stormtroopers around in the Carbon Freeze area but does it anyway to try a last ditch effort to save Han?
Shall we forget that He's got a family back on his home planet? A wife and son? He's out hustling to make a better life for his family. Both monetarily and by trying to make the galaxy a better place.
He then takes off in a ship with a dubious ally to try and save his buddies life.
He then allows himself to be brought in front of Jabba not knowing if he'd be instantly killed. I mean, what would be more heady entertainment than a Wookiees vs Rancor fight?
(No longer canon book evidence used hereafter)
His life debt transfers to Han and Leias children, even for an ultimate sacrifice.
You have to drop a MOON on him to kill him. And he's going to save your kid before that.
Really, how can anyone argue that there is a better side kick than Chewie?