Many players use Teemo as a strictly kiting character and this makes him rather weak in team fights as he is quickly eliminated. The Barrier is directly related to his survivability. I will go through pros and cons of several items I have used and will discuss them with some detail. I tend to shy away from strait AD Teemo as everything he has is based off of AP. Hybrid works well as it gives a tremendous back door ability, which as anyone who has played against a Teemo knows is something he is very good at.

Several things I have seen that works well depending on your play style with Summoner spells.

Flash - Is a must have as Teemo, lest you be caught by anyone you are more or less dead without it.

Secondary Spells

Barrier - Better for survivability obviously, however only absorbs a small amount of damage, for a shorter amount of time.

Cleanse - Good against high CC teams such as Varus, Nasus, Olaf, Viegar, Annie, Taric, Leona, Lux, and Morgana come to mind with this.

Exhaust - Good all around one on one damage reduction spell for engaging and eliminating your opponents while keeping yourself alive. (30% Damage reduction is a lot when your super squishy)

Ghost - provides a lot of mobility and escape, however is rather redundant with his W (Sprint)

Heal - Kinda blanket survivability however easily countered. More effective than Barrier when in team fights as it can save your team mates and potentially yourself.

Teleport - Provided your ability to spread your Mushroom's, this would allow you to be almost anywhere on the map quickly and preform either back dooring with minimal chances of being detected while en route to your destination or ganking lanes.

Banner of Command - This is good for games where you expect to be sitting down by an enemy turret for awhile. Additionally the Siege minion gets you gold, and does more damage than regular minions. All in all, not bad, though not exactly team building.

Blade of the Ruined King - Adds epic damage to high health based champions and gives you a little return in the health department. Great for Hybrid Teemo, Only good for AP Teemo if 3-4 of the enemy champions are tanky.

Deathfire Grasp - This is key to taking out high health targets that you will see end game. The 15% of target's MAX health as well as its 20% buff to magic damage afterwards gives you a great surprise to champions who think you will not do enough damage to them. The combo used here would be proc this item and drop a mushroom on them and then run after an blind and auto attack. This will easily drop them below health.

Frozen Mallet - Great survivability as Teemo, also since you are able to catch most players with ease, this will allow either your ADC, or team to chase them down or make it difficult to be chased if you auto attack as you run.

Guardian Angel - This is just a strait survivability item that allows you to get revenge once ever 5 min's which can be particularly useful in team fights if the enemy believes they eliminated you. The problem arises when you are constantly getting into team fights all over the map, you will likely still end up dieing and will only work ever few fights, though in the event of a closely matched teams, this can make or break the difference of a good one minute ace which allows you to push.

Liandry's Torment - This gives him both additional survivability as well as makes his mushrooms do ridiculous damage. I will go into some additional aspects of his play style and hardships later on. A MUST have for any Teemo Build.

Lich Bane - This is great for moving around, dropping mushroom's as well as quickly taking down a tower. It can get help you get around the map much quicker and either escape your enemies or chase them down.

Maw of Malmortius - General survivability against Heavy AP teams where they do sustained damage rather than high burst, as well as a AP absorption shield. This is good against champs like Ahri, Ziggs, AP based Cho-gath, Morgana, and Lux for instance.

Mercurial Scimitar - This is ideal against champs with high CC like Nasus, Olaf, Riven, Panth, Viegar, and Leona.

Mikael's Crucible - Ideal when supporting against a High CC combo which could include any of the following, Varus, Leona, Lux, Morgana, or Nasus with that slow of his, even a Warwick Jungle.

Morellonomicon - This should only really be used when going against high sustain teams, with either multiple supports. OR combos that involve Dr. Mundo, Nasus, Soraka, Volibear, or Taric for instance. All of these champions have either a large heal, or a ability that restores a large amount of health quickly.

Twin Shadows - Good for helping your ADC chase down a kill or escape, while giving you more movement, damage, and MR. Good to use if an enemy champ is trying to escape your team by running giving your whole team a chance to chase him as well as help track him down by following the little ghosts, great for those sneaky champs that like to trick you in bushes.

Rylai's Crystal Scepter - Good survivability as well as making it more difficult to be chased if you are out of mushrooms or need a second to two to get to one. Blind will now make auto attacks not hit, and slow them down for a 1.5 seconds.

Zeke's Herald - Adds survivability to yourself, gives your ADC an additional 20 AD and 10% life steal. This is also advantageous if you have a AD heavy team when it comes to team fights, it will give them all this buff which results in healing and higher damage output.

Zhonya's Hourglass - This will give you an ultimate survive against quick burst players such as Darius, Zed, Karthus, Vladimir, and Veigar's Ults. As well as a survive in team fights when you get jumped to or targeted.

You will find this build may not have the best mana regen and you will often be running out of mana for dropping mushrooms all over the map, a quick solution is blue buff. Honestly if your not having any issue with living you can also get a mana regen item such as Arch Angels Staff. Though not the best for burst damage early on its effect late game will save your life.