Leona is the boss of team fights and I feel the most fun tank I've played yet. But in order to have fun, she must be played properly, like a TANK. There is no such thing as AP Leona :). (At least no until her scaling is buffed.)

PROS
- Pure Tank
- Great initiator
- Great CC (Crowd Control)
- Good Survive-ability
- Can control lane and zone out enemies
- Passive makes team deal more damage

CONS
- Easily harassed early
- No real damage in laning phase
- Team reliant in fights

*Note I do my best to cover up these cons with the build.

Quints - Flat HP for more sustainable early game.
Marks - Flat HP again, you know, 'cause she's a tank.
Seals - Flat armor to negate auto attack harassing early, and again, she's a tank.
Glyphs - Flat magic resist to help counter those early skill shot nukes, and for TANKINESS.

*Note Dodge yellows are also a good option, but are better late game. Early game harassment is why I choose flat armor, but dodge yellows are not a bad alternative.

I go 0/21/9 for a few reasons:

21 in Defense - 3/3 in resistance and 3/3 in hardiness for tankiness and to counter harassment early. Then 4/4 in evasion to, again, counter harassment. No points needed in Strength of Spirit because her max mana early is just a joke. I also make sure to get full HP and Tenacity.

9 in Utility - Pretty typical, 3/3 in Perseverance for hp/mp regen, 4/4 in Awareness for exp, and I put the last point in Meditation because if you like to use your skills a lot mana could be a problem early on.

EARLY GAME
To start as a tank it's always a toss between Doran's Shield and Regrowth Pendant (into Philosopher's Stone). If you don't have full runes I suggest Doran's Shield, but if you have a full set, it's really up to you, but I still get Doran's for the HP/Armor/Regen combo. If you like to use your skills a lot for protection/harassment, I suggest Philosopher's Stone for the added MP regen, and the Gold per 5 never hurts :). After this grab a Mercury Treads for CC reduction and MResist. I always prefer Mercs Treads over Ninja Tabi, unless they have 4 AD's on their team or something. Also, consider who is fed on the other team, this may alter your choice. Then I grab an Aegis of the Legion for the aura knowing team fights and ganks are going to start soon (if they have not already). This is a must, it's such a great all around item that also helps your team survive. You need your team to survive in fights because of Leona's passive.

MID GAME
The first thing I do around here is grab Glacial Shroud. Leona needs the CDR (Cooldown Reduction) and Mana to stay relevant in team fights, instead of just being a meat shield. She needs these to continuously use her stun in team fights to keep the higher threats out of the fights. Next I grab a Banshee's Veil, which is always a good item for an initiator. Getting carries to waste their first skill on you, without actually being hurt from it, is very good obviously. The added HP and MP and MResist should also help you be more tanky. After my Banshee's I just upgrade my Glacial Shroud into a Frozen Heart for extra MP, CDR, and Armor.

LATE GAME
By the time you get here it's really dependent on who is fed, what type of damage is hurt you/your team the most, and so on. I grab a Sunfire Cap here since I am now always in the middle of team fights. This allows me to damage the entire enemy team, while focusing down their carries. The added HP and armor should also help out. Next, if it is a balanced team, I grab a Guardian Angel for armor and MResist. It is also good if you seem to be dying in team fights, but I never really die much so I sometimes go for an alternative item that can help the team survive more, such as Randuin's Omen.

By now you're probably an unstoppable super tank because you'll have high armor, magic resist, and hp, to go along with your massive CC.

After putting a point in both your Q[Shield of Daybreak(Stun)] and E[Zenith Blade(Gap-closer)] (I put in these first in case of early ganks), you want to max your W (Eclipse) right away. It is your strongest source of damage outside of your ult, and it is also an AoE spell that gives you bonus armor and magic resist, you can't go wrong. It can also be used for harassment in lane against melee champs.

After maxing W[Eclipse], I just alternate between Q[Shield of Daybreak] and E[Zenith Blade]. It's up to you to decide what is more important. Zenith Blade deals a bit more damage is your main initiation tool, but you also need lower CD on your stun if you're going to stay in team fights. If you feel the need to max one before the other it really doesn't matter. I alternate to keep their CD even, since I see them of equal importance. And obviously you put a point into your ult when given the chance.

SKILL COMBO
I thought I'd add her basic combo's here as well.
COMBO 1 - W->E->Q->R(Ult) - This is the basic B&B combo for Leona. Start by activating Eclipse for extra armor/MResist, then close the gap with your Zenith Blade and Stun with Shield of Daybreak immediately. During the stun activate your Ult on top of the person you stun while trying to hit as much of the enemies as possible. (Try to keep the center of your ult on the person you first stunned). By now your Eclipse should have exploded on everyone in range. This combo deals all of Leona's damage in the shortest amount of time. Use this combo to focus down enemy carries right away in team fights.

COMBO 2 - R->W->E->Q - This is the safest way of initiating fights with Leona. Start by ulting over the entire team (or as much of them as possible), while keeping the center of your ult over their carry. Then activate your W and E into the carry, if possible, and stun him/her again with Q. This combo is good in the laning phase to spread out your damage so you can finish people off if your mate can't finish him.

COMBO 3 - W->Flash in->Q->R->E - This is a risky one, but effective if done correctly. Don't you hate when that one person gets away with a chip of HP left? Activate Eclipse and Flash into the enemy carry. Stun with your Q and ult immediately on top of them. Remember how he flashed out after that and got away with 100hp? Well, guess what, E into that lucky guy right away and POOF he's not so lucky anymore. This is good for taking out those tanky DPS players like Irelia and Jarvan

Highly Recommended

Flash - You can use this a part of a combo to close huge caps, can be used to chase and escape. This summoner spell is very versatile and is top tier for Leona. Highly Recommended.

Teleport - It is important that Leona gets around the map with ease and teleport allows her to do so. It can save a tower and help Leona get back and turn around a team fight. Highly Recommended.

Use at own risk

Ghost - Can be used to chase and escape, can't use with combos though. Also it isn't an instant teleportation. Alternative to Flash.

Clarity - If you spam spells early (you shouldn't) this can be considered since she has low mana early game.

Cleanse - This one isn't bad if you feel you don't need Flash to escape.

Remember, your job is to focus down the enemy carries first while protecting your team. Stun enemies to keep them away from your mates if they are low and try NOT TO KS. Let your carries have the kills. Only reason you should pick up a kill is if you're the only person around to finish the enemy off. Also, if you can save a teammate by sacrificing yourself, do it. You are worth less than a fed carry since you would have no kills. Just make sure both of you don't die.

You should let your lane mate have most of the last hits, but while they are busy harassing or if they went back it's a good idea to pick up some of them. Your W[Eclipse] is a good tool for clearing minion waves late game. I tried last hitting early game with Q and E but it is not worth the mana cost.

So, that's it for now, I'll try to update as much as possible. Constructive criticism is always welcome. :) SUPER TANK AWAY! I'll be adding situational builds soon!