Top ability? My vote goes to vigilant (+starbase radius, (all) starbase defense boni+modules, Star forge tree, Weather control zenith tree, and the very powerful arcean navigation centers tree). Basically it would be worth it for the Arcean navigation centers alone, which can make your ships easily the fastest of all races even without built-in engines (tip: watch for adjacency boni where you built the navigation buildings)
I also like wealthy, for the +1 production bonus, which gives a huge boost to manufacturing, research and economy especially in the colony rush phase, where it matters most.