Actually, after his death, I would consider Goku as an immortal too and what you failed to realized is that you're putting limitations on where they can fight. IE. Earth! Ignore those limitations that are restricted to a certain place. We are not comparing fighters here in places where they can or can't fight. We are comparing fighters where and when they would be at the most powerful. Yes, Goku would cease to exist if he continued to fight in SSJ 3 form on earth against Buu, but back in the other world, he would've been fine.
And who saids Goku can't exist anymore after Wu Kong destroys his body. Goku can exist even if his body (in the afterlife) is destroyed. He's already dead and if you've seen GT (I guess not really Toriyama's work), both Cell and Freeza were getting their bodies desimated by Goku, yet they kept coming back cause they're already dead.
I did not ignore the fact that Goku would not be able to defend against Wu Kong going into his stomach. True, Goku's body would be destroyed, but so what, he's already dead (let's assume). Like Freeza and Cell, he'll just spawn a new body. I guess neither Goku or Wu Kong would be able to destroy each other to the state of non-existence, but what I'm getting at here is who's more powerful and to me, I don't see Wu Kong destroying a galaxy with a single blast or whatever.