Character + weapon = cosmic threat
I was thinking about the Keeper, who is Silver Surfer with the Quantum Bands who was transformed (in time) into something much more than the sum of his parts and was able to defeat Galactus. The power increase from adding the Quantum Bands was expodential. Another example of this was the High Evolutionary gaining the power and knowledge from Galactus ship. He grew in size, claimed a herald and flew off into space, the implication being that he had become a Galactus level character. (Of course they never explained how he returned to normal power levels, but that's beside the point.)
So, I got to thinking what character weapon combinations would result in an individual who is not just powerful, but a cosmic great? I would open this to characters and weapons of all comic universes. I'll start off with a couple:
Tutinex w/ yellow ring-
His power is that he can instill fear in others and pulled off the incredible feat of installing fear in Thor during battle, something that just does not happen. Physically he's already in the elite tier.
Sentry w/ mind gem-
My theory is that he could stabilize his fractured psyche, full understand his power and because a universal force for good....or bad. He aready managed to make the world forget about him!
Who else ya got? Remember to say why or else it's not worth it.