If you can have only one, go for the Hunter. It's by far the best fighter in the game, it doubles as an effective anti-tank platform, and it's much cheaper too points-wise, even with the exarch. Standard pulse laser + bright lance + exarch works wonders.
Having said that I've been using only the Hemlock in the past few months. The combination of two D-scythe blasts + mastery level 2 + psychic powers makes it a strong choice. Then again, my current army's anti-aircraft role is filled by Dark Reapers and Swooping Hawks. So it depends I guess.