One of the most common clan castle requests is for defense. What makes a good defensive troop?
(1) attack fast, do lots of damage, splash damage preferable
(2) live long and soak up attack damage
Dragons are the best defense troop IMHO. They have high hit points, attack reasonably quick on both air and land, and do lots of splash damage. Wizards and valks are also awesome. Valks can mow through legions of small ground troops or wipe out an army of giants in no time flat. Wizards can defend against air troops and do a decent job against ground.
Balloons are devastating against giants, barbs, and valks but die quick to people who can attack air troops and are useless against air troops. Minions seem pretty good.
Watching competition matches lava hounds are very popular. This is because they take so much damage and then spawn lava pups when they die.
Archers are fine but die quick.
What NOT to donate for defense: Giants (slow weak attack), Golems (ditto), Goblins (duh), wallbreakers (duh), Barbs (not that strong, useless against air), miners (their value is avoiding defenses), hog riders (weak), Bowlers (ok but die quick), healers, Pekka
What you should donate: Dragons, Valks, Wizards, Witches, Archers, Lava Hounds, Minions, (1) Baby Dragon (good with valks to provide air cover), and balloons in a pinch.
The easiest way to remember is donate the top row starting with the wizard. You can only go wrong with the bowler in that case, but the bottom row is almost all bad for defense except the archers and lava hound.