Swamp Fox Clash of Clans guide to upgrading:

Note: rushing can be very detrimental to your clan.Do not rush or you will regret it and you will hate the game.

First thing you will want to prioritize in upgrading defensively is your splash defenses because they hit multiple targets at once. You need to upgrade your splash damage weapons first. By this I mean weapons that do damage to multiple troops at one time. These weapons include your mortars and your wizard towers.
Why upgrade these first? Because they can single handily win you a defense against an attacker. Like I mentioned above, they can destroy multiple enemy troops at once making them an awesome defensive weapon.Next upgrading your defense weapons that can attack both air and ground troops is a good idea. These weapons would be your archer towers and your teslas.

You upgrade your splash weapons first, and then other defense items. Keep the upgrades going in line. Do not upgrade wizard towers twice to any other. Also, keep upgrading until you are told you need to upgrade town hall. At this point you upgrade your castle, then your walls until walls are maxed. You also want to upgrade troops until your laboratory tells you that you can not upgrade anymore. When all items are maxed, including walls, then you upgrade town hall and move forward.

The wizard tower is overall a more important defensive building because it does damage to ground and air units.

 This helps when getting attacked by minions and in some cases even balloons because they sometimes all bunch up near defenses. Wizard towers also do more damage per second. Yes, the strength of the mortar is that since its wait time to fire is so high it'll tear out all tier 1 units in one strike, but it being slow also causes it to miss sometimes because troops run. A wizard tower shoots faster and can at least make a troop weaker for a point defense to take out. Wizard towers can really do quick damage to any goblins, minions, barbs and archers eating at your storage's.

Air defenses are next on the list of importance.  you don't want to be vulnerable to air raids. If you have low level air defenses, you're going to be getting hit by balloon-minion and dragon based raids at higher levels of game play, and healer-giant raids at lower levels.