Well, when I was still toying around with who I wanted to main, I was ******* around with R.O.B. He seemed to be pretty good (this was before people knew much about metagame), and he was fun to play. I watched Overswarm's R.O.B. vids and thought they were amazing. I was convinced, I wanted to main R.O.B. I kept watching videos until I saw OS lose to cosmo and the like when playing G&W. I tried out g&w and immediately liked him. His maneuverability is absolutely amazing. Any time you are in a sticky situation, up +b is a pretty good choice.
Also, in my opinion G&W is one of the best characters. He isn't a say, hard, counter for a lot of other good characters, but he certainly makes a large amount of characters strengths disappear. (Example, G&W vs Zelda, ness/lucas, etc)
I've seen people argue about tiers, and I see G&W often times 6th or 7th down. I don't know every character's current metagame potential (I'm not that much of a smashboards beast), but I would be surprised if that many characters could surpass him. I see the possibility of metaknight and snake being ranked higher, but what other characters can beat him/have more good matchups? Pikachu maybe? Really good marths can be troublesome as well...
If someone wants to enlighten me as to why characters that I didn't mention are better than G&W, I'm happy to listen.