A lot of AD -- Thornmail. Thornmail is kind of a no-brainer, specially against agi. champions such as Master Yi and Jax; if you have good HP, you will have them raping themselves. Not of much help if you're also a squishy, however.
Randuin's Omen is decent, as well, and pairing it up with Banshee's Veil will have you protected against the worse of both worlds.
Aegis' Legion is a good item, too, considering how it's relatively cheap and can be acquired in the early-mid game stage, only losing its usefulness at late/end-game phases.
Mercury's Treads is more impressive than it looks; the 35% reduction goes ways into saving your a**, and you will thank god for having one when you're getting chased by an escorted Ashe or a skilled Twisted Fate.
All-in-all, defense items are *very* situational. It's hard to find a team that doesn't lean one way or another (AD - AP), but generally items that give de-buffs to enemies, buffs or increase your health are very multi-situational.
Guardian's Angel can be a good item, but unless your champion has escape abilities (Guardian Angel can be ridiculous with Shaco -- if the team doesn't spam CC and AoE right before you respawn from GA, you will escape), or a good team pushing the enemy off, you'll just revive to get killed again.