1st Ship:
Pretty cool. A little on the circle-UFO side but that can be good. Floating Aegis protection balls are pretty ugly. The stretched missile launchers look pretty ugly.
Touch it up a little with a (Possibly) better shape and remove the balls.
2nd Ship:
Gem of the fleet. Fearsome little beast. Looks like it could do a fair bit of damage. Only problems is the yellow center-line that runs down the front middle, The little arm bits just behind the side-pods, hap-hazardly placed engines (Parts of the ship behind them. Because you love burning holes in the sides of your ship), the exposed core, stretched weapons, those annoying holes in the hull of the ship (Check the sidepods) and use of large, ugly and flat sections.
Remove and fix the noted problems and touch up the colors and shading.
3rd Ship:
Pretty nice. Second best one. The stretched weapons, exposed core, annoying empty centerline, holes in the ship and that same circle core thing.
Fix the mentioned problems and it could pass. It's still a rounded triangle though.
4th Ship:
Ew.
Heinous green color, use of large, flat and ugly sections and worse yet they're repeated in hideous fashion, Use of mechanical and square sections and then some curvy, alien sections, Core is placed at the front rather than near the middle, Ugly and unplanned shape and a general mess of untidy sectionwork.
Ditch this fucker. It's almost un-salvageable. Possibly the idea can be re-used but... Bleh.