In most cases it is best to throw down a card second so you can effectively counter whatever your opponent throws. Learning what counters each card best comes with time and experience and you can tap into some of that knowledge below.
Balloons
For me, the most effective counter to the balloon is Minion Hoarde, however that card is very susceptible to arrows, so you may have to have a dragon in your deck also. However, if you drop Minion Hoarde directly onto the balloon it may well destroy it before your opponent gets a chance to throw arrows or zap your minions.
Giants, Prince and Hog Rider
A really effective deterrent against these tower destroyers is to place another building, centered in the playing field and slightly ahead of your towers. This should mean that, as long as they are not already engaged with your tower, they should head in the direction of your new building (canon, bomb tower etc...).
Giant Skeleton
He's pretty easy to take down. Minion Hoarde will easily do it... But my favourite method is to drop a set of Barbarians directly BEHIND the giant skeleton, that way they will escape from the bomb that the skeleton drops.
Skeleton Army
The best defense against these is the Zap - It only costs 2 elixir, otherwise Valkyrie works really well, as do the Spear Goblins.