Xerath always called the most powerful champion in lore-wise but Bard has came. So, what do you think about them?
I pick Xerath because may be he cant manipulate time and space but he still has more overwhelming power I believe.
Eventually, he will top them all off 
. (Nobody expects the Ascendant Shrimp)
Power wise, ofc Gods are always the most powerful, the only problem is they probably won't or can't use their power for destruction purposes for whatever reason, personal or otherwise.
But if we simply evaluating sheer power with everything considered, Bard is the more powerful. He can travel between space with ease bro, like literally severe the dimensions, reattach them in some way to create a portal and it's even stable enough for everybody to use it. And manipulation on time itself too.
Xerath just has loads of energy (still the amount is leagues lower than Bard's) and has nothing to do with them besides throwing them around, pretending to be bad arse.
Imo, between the 2, Bard is stronger.
Bard can't travel through time either,just through space(seriously why is everyone assuming that once you can travel through one of them,you can travel through both?)
If Xerath was unbound,then I'd say Xerath,but I guess they would be even if Xerath was still chained by his sarcophagus.
