Upgrade Mortars and Wizard Towers First

Splash damage dealt by the Mortars and Wizard Towers are your best defense against the main troops used by most players in the lower trophy levels of this game. These troops being Barbarians, Archers and Goblins.

The splash damage is not concentrated to one single unit so it is able to take out large groups in one hit.
Because of this it is always a good idea to upgrade your splash defense towers first. Mortars are lower cost and have greater range so it is a good idea to start with them.

Wizard towers are more expensive and have much less range, however they do attack faster and with more damage.