My interlinear Bible suggests that "a root of all evils" is the most literal rendering, but I won't pretend to know the workings of ancient Greek. The King James Version gives "the root of all evil," while the NET Bible argues for "the root of all
evils" (plural; emphasis is of course mine).
See footnotes 12 and 13 to
1 Timothy 6 in the NET Bible, particularly the latter: "Many translations render this 'of all kinds of evil,' especially to allow for the translation 'a root' along with it. But there is no parallel for taking a construction like this to mean 'all kinds of' or 'every kind of.' The normal sense is 'all evils.' "