Shen in my mind is one of the best if not best tanks. He can save people with either a taunt or his ult, or a combination of both. If his ult is up he can save people from any lane which puts him above other tanks in that regard. It is the job of any tank to save their teammates as well as initiate and he can do this from anywhere on the map. He also has a jumping aoe skill shot taunt which when done right can take multiple people out of a fight, and initiate. Also always remember as a tank it is your job to help kill but not get kills, if you accidentally do ok, but try to let the carries get kills.

Furthermore as an important tip for any Champion remember 15 minion kills nets one champion, wards prevents ganks, and dont chase someone if they run it means two things you can beat them whether it is now or later, and you get free farm to get loads of gold while they are back in base trying to heal then run back.

Pros:
Decent early game damage q for harrasing.
Aoe taunt
Shielding ult which teleports you to that champion no matter where they are.
Shield skill to help tank turrets, block dot skills and anything else you want to block.
Can regen health fast early game due to the regen on hitting whoever he uses his q on.
Passive that can do high amounts of damage early game when harrasing.

Cons:
Taunt uses over half of his energy so you must be aware of your energy usage.
Does not do much damage late game compared to dps tanks

You can use whatever runes you wish but i choose armor and magic resist marks to give me some added defense at the beginning. Seals of health per level to give me extra health as well as quintessences of health. And flat cd glyphs to reduce ult cooldown which is very important as well as taunt/q/w. Cooldown glyphs help shen tank better, faster, and harder.

Good alternative runes could be more armor and or magic resist for seals. However i believe cooldown runes are very important.

I do 1/21/8 for shen because i feel that it is important to get the extra experience from the utility tree, and a point in exhaust to increase its duration as well as make it lower their armor and magic resist. This can greatly help early game to keep them exhausted longer to do less damage, take more damage, and keep them from getting away longer. Plus it allows the tower to do more damage (if you taunt them by the tower) for longer. I also feel that the 21 in the defense mastery tree helps him tank better by increasing resistance dodge and decreasing dmg.

The core items that you should always get are merc treads, unless low amount of ap and or cc and high amount of ad. Always start with dorans shield. Build aegis next for added armor magic resist and an aura that helps your carries sustainability in team fights. Once again any way to make your teammates take less damage by taunting or items is always helpful. Plus it gives them a little extra damage which helps early game. Then get the health for sunfire and finish it before getting banshees. Those round out the core items i get every time. After that it depends on how heavy they are on ad and or ap. If ap heavy get Force of nature, if ad thornmail. Then Guardians angel to round it out. You can get guardians first if you just want to get both armor and magic resist or an extra life.

The skill sequence you go really depends on what you team composition is, what lane you are going to and if your team has a jungle. If you go bottom lane get taunt to max first before your q to help your teammate. If you are top solo max q first before e then w. If you have a jungle save picking the skill until you see if the opposite team will gank. If they are ganking get your taunt to take dmg and keep them off your teammates. If not get q to help leash and help your jungle regen some health for hitting the the minion you used your q on. Remember your q helps you regen health fast along with dorans shield so use it when you can to help regen your health or teammates when enemies are not around. Or on the enemies early game to do dmg and help your or your teammate attacking regen health while fighting.

Like i said before exhaust helps cripple an enemy dps, or keep someone close to kill and keep them from running away. As well as increases the dmg they will take by a little. It can also keep them close to a turret when taunted. I also take ignite to stop the enemy from healing secure kills if need be, this is also good on champions like tryn when he ults to attempt to kill him once its off if he runs away. Also it is good to note that ignite reduces all healing by 50% so lifesteal is reduced by 50% as well.

Remember
Buy a ward.
Stop a gank.
Save a life.
15 minion kills equals a champion kills
When they run away from a fight in lane phase it equals free farming
Kills are not important, not dying is important.
Turrets and Dragon are worth 2.5 champion kills or .5 for each member of your team

Here is some of my ranked play with shen. I have an 80% win ratio. I am currently 18-4 with him in ranked at the time this was made and will add more photos as time goes on of how i do. In this photo i played top solo against singed and killed him 3 times up top, and a couple times the opposing xin attempted to gank, and my taunt exhaust combo got him killed when he attempted to tower dive. I also got kills accidentally when trying to save teammates. I will admit however that even though we won this game, and i did well i did not do my job as a tank as far as letting them get kills. I did not try to get many of the kills i did get but i did do well and helped my team. Also remember that when you play shen there are many times you save people that will never show on a scorecard.