I filled an Ortlieb large rack bag with water and not a drop escaped.

for Ortliebs.I tried the Aldi alternative to have a set for getting muddy at the allotment and the hooks broke within a month. Hopefully they've improved that part for future versions.Ortlieb are great if you can afford them. I used panniers from Lidl for years and they can withstand the pissing rain that makes Glasgow so green. They are rolltop like the Ortliebs but don't have the fancy adjustable mountings nor the spring-loaded fastening. However, they are fully waterproof and at £9.99 they are well worth the money.
GC
Probably not. With the cheap ones, I'd go for the hanging over double type. Less to break so more chance they spent on better waterproofed fabric.I tried the Aldi alternative to have a set for getting muddy at the allotment and the hooks broke within a month. Hopefully they've improved that part for future versions.