Though I started with Mountain, I now find Sabre to be the one that works best for me. Yes Mountain does slightly more damage with acid and blood leach helps with survival, but only to a point. To use the Mountain Dragon efficiently he needs to stay alive and keep hammering the enemy, however you could get overwhelmed and all the advantage is for naught.
Sabre has one big advantage and that is Sabotage, it's so good early in the game. Keeps Transports from unloading and factories from producing units. I found myself often spawning above my units, using advance to speed them up, then jetpacking to enemie base, disable the factory, jetpack out and doing the thing all over again, works pretty well.
Then the hunter is imo the best early unit pick, not because the other ones are bad units, but because you can produce 2 movement point units in your first turn without spending research points.
Still, when it comes to optics, nothing beats that Mountainn flail-tail.
