As engineers use intelligence and psychic abilities, engies frequently wear Int/Psy armors such as cyber armor at low levels or nano armor during the mid-levels. Carbonum armor is also useful because you can make it yourself. While you're at it, hunt down a couple of robots and make Junkmetal shoulderpads as needed; there's no reason not to, and they add a bit of max health.
Shadowlands users have the Jobe Suit for engineers and Control Unit armor available to them; neither suit is particularly useful in combat, but they have some decent tradeskill bonuses.
At high levels, the Dirty Bioplast Coverall, Helmet and Breastplate of Technical Ceremonies are useful tradeskill buffing items.
Other useful armor for an engineer is the Ring of Presence since it gives nice bonuses in nano skills. Obtaining Reanimator's Cloak in the Crypt of Home since it also gives bonuses in nano skills.