I know it's hard to quantify who's stronger than who but quite honestly I think Tathamet takes this easy. Sargeras can effectively shape worlds with his power but Tathamet fighting Anu created the Diablo Universe. Who do you think would win and why?
Sargeras: Most powerful champion of the Pantheon prior to Fel empowerment. Completely immune to fire damage and highly resistant to magic damage and physical damage caused by mortal means. Has been slightly harmed by immortal weapons. His strength is nearly equal to a portal closing. Sargeras is said to be a huge being almost beyond comprehension
Tathamet: Powers of the Prime Evil and every single demon in hell combined as the sum of all evil. Survived in the space devoid of matter prior to the "big bang" caused by the final clash between Tathamet and Anu. His remains currently make up the Burning Hells, and his heads became the Prime Evils. Tathamet's size has not been explicitly stated. However, the Crystal Arch appears to be several hundred feet tall. Its size, (Anu is portrayed as being miniscule in comparison to Tathamet) coupled with the fact Tathamet's remains are equal to or larger than the Burning Hells, it can be speculated that Tathamet is likely similar in size to a medium to small planetary body.
My reasoning: Due to the fact that Tathamet could survive in an environment that is completely devoid of matter, it is likely the laws of the universe may not affect Tathamet, as well as the fact that only a being comprised of all the "good" of the universe could destroy him. This alone took place in an seemingly unending battle that lasted for millenia.
Some info on Sargeras and Tathamet: http://wowpedia.org/Sargeras http://diablo.wikia.com/wiki/Tathamet.