The saga that shows what Goku is about the most is the Red Ribbon Army saga. Goku sets out to get the four star ball that his grandpa had as a keep sake to remember him by. Then goes around using dragon radar to find the balls. The only reason he's doing any of it is to find his grandpa's ball. In the mean time, he kicks Red Ribbon's ass because they get in the way and try to stop him from getting the dragon balls, not because they're evil. Goku doesn't care. Goku just wants that dragon ball. The only time Goku stands up to evil is when they hurt one of his friends. It's not that Goku is an asshole. He just places fighting above being good. But he's pure hearted in his desire for strength. Goku has never been above that. Any time he deliberately confronts evil, it's always because someone hurts someone he cares about (Taopaipai hurts the Native family, Piccolo kills Krillin, the entire Saiyan saga, Freeza kills Krillin;etc.)
My take on Goku is a guy who seeks power and strength above all else in order to better himself as a martial artist. That isn't to say he's power hungry or he's malevolent. At the end of the day, despite priority number 1, he's still a guy with a heart of gold. He cannot stand hurting others willfully and likes to help people, but that will always be priority number 2. This makes Goku a great foil with a lot of characters. Whether it's his son who is the exact opposite (priority number 1 is the people he loves, no 2 is fighting), or his greatest rival (also loves fighting for improvement but never for the pure, innocent goal of getting better but to prove his superiority).
I personally have always found his character to be great and refreshing amid an assortment of heroes who always care about Doing The Right Thing (tm). The fact that Goku is almost always incidentally a hero is one of the reasons I find him appealing. That said, DBS does at times Homer Simpson/Ned Flanders-izes him, but he's still Goku being Goku.
And that's all the passionate Dragon Ball posts I can write late at night/early in the morning because I can't sleep...