Each and every spell has its strengths and weaknesses, it all comes down to how you like to attack. With the way the updates keep coming, you can expect the creation time to decrease each time. This means you should be setting up your base to defend against these, or have the knowledge to use them properly.
Lets take a quick look at the lightning spell. A favourite among players, this spell has can be used in a variety of ways. Firstly, it's capability to take out a defense when paired can help win you the battle. I've found mortars to be a favourite target, what with their range and power. Lightning spells are so handy for taking out those pesky clan castle troops bothering your own. One lightning spell can take out the troops and save you many in the process. Another thing that I'm sure many higher level players such as me are frustrated about is the natural attraction of lightning to dark elixir. Many low level players have a drive for the stuff, so if you're saving, expect to lose no matter how heavily it's defended.
Second is the healing spell. Unlocked at spell factory level 2, i have found this to be an overrated spell. You may argue that it's capability to heal a swarm if giants, or protect your barbarians from mortar or wizard fire can be the difference in the battle. However, this spell can be easily replaced by the healer. The spell can be used to help prolong that dragon or pekkas life, while you get that extra 50k gold.
Next is what I would say is the most used spell in the game, rage. Unlocked at spell factory level 3, this is probably my favourite in the game, an if you talk to many higher quality players, they will all agree on its necessity for victories. The rage spell makes your troops move faster and hit harder, and when used properly, will win you that 100k you need for that upgrade. All troops are used effectively, but a swarm effect will give you the most bang for your buck.
Finally is the jump spell. Unlocked at the final spell factory upgrade, this is the most unused spell in the game. Firstly, it's fairly common sense that this spell need not be used with air troops, who just fly over the walls. Secondly, why jump over when you can just go through? 4-5 wall breakers will take the place of this spell no problem, saving you the cash required to make it.
I hope this helped you out deciding which spell to use in your next battle, and gives you an idea on how high of a priority your spell factory truly is. Remember, the quick creation times make using spells a privilege, as they can be used almost every battle. Have fun and clash on!