Akkan is the best but for some reason I always build a Kol first. Colony ships just don't have enough antimatter to colonize planets fast enough against a very aggressive AI (the Akkan does). I generally build my first capital based on which one I want the highest level first. A high level Kol paired with a high level Dunov is a mean pair. The rail gun and flux field do a lot of damage(but not AoE) and the Kol is a fairly tough ship(I think it's max armor and hull are the highest besides the Corsev?). The Marza has a very good "ultimate" ability with it's missile barrage(very good AoE). It depends on your tastes and fighting style.
While the Kol and Dunov are a great pair, having a strategy that relies on 2 high level caps is not usually a viable one competitively. Besides, with the TEC Rebels you have something far better: the Ragnarov and a lvl 6 Dunov. Snipe kills Gauss Rail Gun in every way besides a slightly longer cooldown, and its something you can pull off with a level 1 titan. Plus you can use it from far away, making it safer for your Dunov.
Also while the Kol probably is the toughest one, base stats are not what really determines the "tough" capitalships. The ability of a cap to heal itself is far more useful than its base stats, hence why the Corsev and Kortul are great tanks thanks to Salvage Teams and Power Surge. The Kol technically has one with Finest Hour, but again that's a level 6 and the Marza or Corsev would be far more useful in its role unless you're fighting tons of strikecraft (which you won't early game).
