So far my favorite ship design items are Planet Bound for Millenia and Auxiliary Plasma Banks. With Planet Bound, you really get so many awesome goodies cheap. You get the modified drive for space cost only, you get one bit of Archeaotech, for the trade off of -1d5-1 Hull Integrity (-1d5 for LC and Crusiers). You also get +10 Man near a planet. The modified drive gives you a speed +1.

Auxiliary Plasma Banks give you more power, and cost a little space. They might blow up on a critical hit, but it is worth the risk.

Anyone else have favorites, and why?

Probably the Mining rig. Yes it does take up a hell of a lot of space, and is probably only a sensible purchase if you have more than one ship, but it does allow you the flexibility of doing a background endevours without having to keep purchasing a whole lot of kit. Onboard Refinery. Trackter Beams. Bays of Mining Lighters. Holds for special materials. With a bit improvisation you can turm this kit for other things (ie you can train up the crew to peform salvage operations) On a big enough ship you could probably buy a munitorium and keep out in the expanse for years, mining the materials you need to keep you ships equipped. If you get yourself a couple of Chiropatean scouts, increase the auger detection and a couple of ore seekers you have a ready made platform for ecomomic exploitation and you dont even have to rely on shifty third partys. This allows you to concentrate on exploration and conquest if thats the general direction of your game.

A word of warning though. As our group has found out on a transport you quickly run out of space. Our Brand new ship can only manage a dorsal marcrocannon and reinforced bulkheads. Enough to make a pirate think twice but not to see him off.