After trying all sorts, our volunteer group has found the hori hori knife to be the best all round tool for tackling the most common perennial weeds (veldt grass and gladioli) in our bushland reserve in Perth. It's relatively expensive, but is well balanced and robust, stays sharp, is a pleasure to use and will last a lifetime.
Just wrap some red electrical insulation tape around the handle to make it easier to spot if you put it down during weeding.