Galactus created Silver Surfer as his most powerful herald. After Surfer left, he made it a point not to make future heralds more powerful. Morg tops Surfer because his cosmic power was enhanced shortly after by some well he came across while looking for a suitable planet for Galactus.
The Airwalker shouldn't be third on the aforementioned list because when Galactus was contemplating about a new herald, shortly before Morg was chosen, he wanted someone who was without "the vulnerability of Airwalker", "the lack of devotion of Firelord"....etc
Anyway, back to the topic, Beta Ray Bill, who's supposedly on equal power terms to Thor got wasted by Morg, similar to Terrax. Morg was an executioner and Terrax was a conqueror, in their fight, it showed who had the better fighting skills. Seeing as Thor is a warrior, and as cosmic power and asgardian power cancel out - he comes out on top of Terrax.
The Destroyer was only a herald for a short amount of time (only about 2 or 3 issues). Thor gave the destroyer to galactus without knowing what it's true purpose was. Loki then stole it back from Galactus. Odin then used the destroyer armor against the 4th Host of Celestials. Since then, the destroyer armor has been in and out of the thor stroyline.
When the power of cosmic was animating the destroyer, it actually brought the Destroyer down in power. In fact so much, that the Thing of the fantastic 4 was able to beat the living hell out of it