I think Nynaeve is arguably the second most important character in the series, here's why:
Saved Egwene (future Amyrlin) from the Seanchan.
The first in the series to use balefire.
Bests Moghedien in Tanchico. retrieves male Adam, and seal. from out from under Lindrin's nose.
Captures Moghedien.
Instrumental in killing Rahvin.
Heals Stilling.
Beats block, marries Lan, uses bowl of winds, and saves Kin and Wind Finders from Seanchan.
Cleanses Taint. Rand was channeling Saidin, but was passing it through Nynaeve' s Saidar filter.
Heals ma