Flames, sparks, and frostbite are getting very weak now.

Im currently using fireball on the left and healing on the right, but i prefer having double attack power.

Also, just in general whats like... the best spells in the game?

UESP is your friend.

http://uesp.net/wiki/Skyrim:Spells

After the spray spells you get the bolt spells like firebolt, ice spike and lightning bolt. Then you get the aoe spells like fireball and chain lightning, however they have bad magicka cost/damage against single targets. The best spells are the expert level spells like incinerate and thunderbolt and icy spear. Most of the master spells are bad, the only one I use is Lightning Storm and only against dragons because it has a huge range and is easy to hit them with since it is a constant beam you can aim around. Well actually, I also use it against dragonflies and bunnies for same reason.

go to the court wizard in the 'castle' of every major city to buy new spells

So i should go for incinerate, thunderbolt and icy spear and after that lightning storm?
The fireball is fairly bad for magicka, i agree...

With my last character the AOE and wall spells were great.