Well, what would the the ring help with that the Power Cosmic already can't do by itself?
I don't think it would somehow stack in power as some might say...I'd just wager the ring would become obsolete. Maybe a Lantern's experience with the ring would allow them to be more quickly at home with the Power Cosmic. That might be one advantage.
If I were a GL, or any other sentient being, I'd never be a Herald to Galactus, for one he screws with your soul, and for two, you lose your genitals. You can't tell me that Norrin still has his boys.
To answer your question though Kilowog would make the best physical specimen, because without the ring, he is still super human, and the PC would most likely make him more powerful physically than any average Green Lantern, barring Sodam Yat (is he dead or alive?)
I would think that the ring would help make the PC more dynamic in it's usage, much like the Quantum Bands, maybe to the point that it would amplify the GL/Herald.
I'd definitely go with Kilowog as my choice unless Sodam Yat is still alive. yat would be the most powerful Herald ever imo.