If you get a science facility and science vessels, research irradiate to use against the zerg and emp to use against the protoss. Ghosts should have cloaking and lockdown researched at the very least and the eye implants researched if you plan on using nukes. The Yomato gun is a valuable upgrade for your cruisers. Although it takes a long time to recharge - it does give you a few powerful blasts when you need it. The Terran Defense Terrans have the strongest defense of all the races.
Bunkers are formidablethroughout the game and can stop numerous invasion attempts.
Missle turrents provide powerful anti-air defense as well as detection. Siege tanks provide extremely powerful ground fire on advancing forces. The three when used together are nearly unbreachable. To setup a great defense you need to consider your opponents. If you are playing protoss or zerg you will want to have at least 1 firebat in each bunker.