Depends on my situation and what I'm defending against.  The best defensive structure is, hands down, the repair bay.  Not only does it improve every other defensive structure by making them harder to kill, but it also provides a place where you can retreat a damaged fleet or capital ship to be repaired.

I usually don't use turrets.  The reason is that their range is poor and they are too easy to go around or just kill with long range units.  They're not worth their heavy price.  I use them to defend repair bays and other key structures; on their own they are practically worthless.

Couldn't be more true.  While you occasionally get a random map with some very tight chokes, most of the time it's not possible to create an adequate defense on all your borderline planets, so without a good offensive to keep the enemy's main fleet pinned down you're going to get outflanked and annihilated.

Oh definitely!  The AI has a bad habit of running head-first into your defenses.  While in Entrenchment it finally learned to move its fragile siege frigates around your planet to a defenseless angle, it still rushes into tightly packed static defense rather than staying back and pounding them from afar.  Because of this, against a player you need to spend more frugally and rely on your fleet more for defense.  This also applies to starbases.  You can pack a starbase with attack and defense upgrades against an AI and it will run straight into its lethal gunfire, while a player will sit back and never give it the chance to fire a shot.

Thanks Darvin!

wasp, where is the hold position button again?

Oh, I see! Thank you very much. ^^

Everyone talks about how a well balanced fleet tends to be better than spamming any one unit (in most cases, stay with me here).  I think the same applies to defensive structors around a planet, at least against the AI.

Computer: As TEC and Advent I like to build my defence structures around my starbase.  I put repair platforms so the starbase is just barely in their range, and then I place point-defence platforms right around them to help defend them during an attack (make sure to protect the planet with Aux Government or Enduring Devotion via the starbase).  Point defences can't do it on thier own, but niether can any other structure.

As Vasari I place point defences as close to the planet as possible, going around the planet, so that the entire "surface" of the planet is within range of one.  This protects the planet against AI-owned siege frigates and pirates, but little else.  Usually 6-8 'guns' will cover the planet; only 2-3 are needed for asteroids.

Human: Against the AI, on any level, a well balanced mix of point-defence, hangars, and repair platforms will do, especially if you mix them in with a starbase.  Against a human opponent... good luck.  Unlike our unfriendly dunce of an AI, a Human opponents can learn, and come up with a solution to any stragety.  An Advent player could culture you out, Vasari could skip to anywhere else they want (or launch their own SB against you), and the TEC could come in with a missile-barraging marza and a buttload of orogov's and hoshikos (a few basics just for starters).  Against humans, you need to be as adaptive as your enemy, you might even need to scuttle/change you defences midgame.  There is no one best trick.