you can win a battle, but it doesn't mean you've won the war. 

for that reason, teutoburg forest is actually the most substantive battle in this list, as it established a boundary between the roman and german realms, and if you want a real turning point, i'd actually point to it rather than adrianople. i know this is very early for a turning point, but it was a turning point towards stagnation, which is the precursor to decline. at teutoburg, the romans lost the war - whereas at cannae and adrianople, they merely lost battles in wars that would continue on.

the goths caused a lot of problems, but they could not hold land in the east; this was a lost battle, and not a lost war.

so, when did the romans substantively lose ground in the northern balkan areas? it wasn't really until the slavs started pouring in, and the various turkic groups followed them. unlike the goths, the slavs made the land their own and pushed the roman boundaries south.

that said, it was perhaps a turning point for the germans who could now take the field with a higher degree of confidence, and less fear that they were fighting a superior foe. and, that goes back to the stagnation i previously referenced - it's not like the germans were technological juggernauts, all of a sudden, but their slow rate of development outstripped the stagnation that beset rome the day that caesar took power and declared a dictatorship.