All will fall to viktor and the power of science!!!
On a more serius note, Karthus is the strongest mage in lore, due to him having a f***ing song that obliterates the world. 5 seconds kills every champ on the map. The entire song might destroy existence itself.
As for the 3 available, I think ryze wins. As stated xerath, so long as he remains caged in his mobile tomb, is limited in power. Not very limited, but still limited. Syndra is similarly limited by experience in using her power. Xerath seems to one who can direct archane in a spot, but not do much with it after. He's kind of like a weaponized testla coil, and a very smart and acurate one at that. Syndra just seems to, due to lack of words, noob-ish with her power to match ryze.