Why play Leona over other tanks?
That's an easy question to answer. She's a monster who has one of the highest amount of crowd control in the game, not to mention when you add her passive to your carry's damage she becomes a force to be reckoned with.
I bought Leona when she first came out and played her for a while until the Talon came out and at that point I was hyped over Talon so I put her on the backburner and never really played her until a friend of mine played her and I noticed all of her potential again and fell back in love! So I got on the sites and checked to see what builds everyone else was running and then I put together my own setup which worked for ME, and this is not to say it will work with you.
I'll explain why I chose the runes/mastery points that I did later on in the guide chapters, but for now just glance over what I have up there and make your own assumptions. Here's a picture of a few normal games that I ran with a friend of mine just to show you how well this setup has worked for me.
Masteries
0/30/0
Ah, yes, the masteries. You may ask yourself, "Why does he go all into the defensive tree?" Well that's an easy question to answer. You're a tank! You're not only getting the extra gold from the assists and kills you gain but you also reduce the damage done to you and gain more run speed to help with your initiations.
I have thought about a split between the Utility/Defensive tree, but I am yet to gather the balls to do so. When I try it out, I'll edit the guide and post my results but until then I'm sticking with my 0/30/0 setup!
Summoner Spells
Heal and 
Flash without a doubt! The heal gives you the clutch ability to save your carry and keep them in the fight long enough to get a kill or to get away without the danger of dying. That moment when they tower dive thinking that they can take you on is quickly rethought when you hit Heal and they **** their pants when you stun lock in your turret's range with nothing they can do about it.
Leona is all about taking advantage of your enemies mistakes, so remember that.
Items
Pick up a 
Regrowth Pendant and stay in lane until you have enough gold to go back and build it into a 
Philosopher's Stone which you can later turn into whatever you want.
I choose to go with the 
Ionian Boots of Lucidity over the 
Mercury's Treads due to the fact that the more you can use your spells, the more damage you can help add on and the more CC you will have and you can never have too much CC. Ever. HOWEVER, if you notice the the other team has as much CC as you do and a lot of AP or a good chunk of them are AD then go ahead and pick up those Mercury's Treads or 
Ninja Tabi! There's no sense in beating yourself up as you can always get your cooldown reduction through other items.
The way I run my setup is I look at what the other team is building because as a tank you have to react to the other teams setup. THERE ARE NO COOKIE CUTTER SETUPS FOR TANKS! I honestly don't think I can stress that enough. I will hardly ever have the same items as the game before or the next game.
I like to pick up 
Heart of Gold for the fact that I give up ALL creep kills to my carry as they can use it more than I can and they don't have room for items like Heart of Gold or Philosopher's Stone in their builds. In the end, it's the best. You can always sell these items if you don't want to turn it into anything later as they would have payed for themselves by the end of the game.
Also, let me add this... AND I CAN NOT STRESS THIS ENOUGH! Buy health, health, health. Once you've reached the 150 mark for both your magic resist and armor, it's really pointless to keep stacking it as your 
Eclipse spell is giving you hellacious amounts of armor and magic resist making you into a super tank for up to 5 seconds. I wouldn't steer you wrong, people. picking up an early 
Warmog's Armor or 
Sunfire Cape can really help you out in the long run. I prefer to go with a Sunfire Cape as it gives you armor, a nice chunk of health AND some added damage for when your team just ISN'T doing enough.
I'll include a list of items for different scenarios to give you an idea of what I'm talking about. Again, these are just EXAMPLE lists of items you might want to pick up when you notice the enemny team's composition, these are not to be the end all, be all items. As I've said before, tanking items are situational.
AD-Heavy Team
AP-Heavy Team
A Well Balanced Team
This is to say that they're 3 AD/2 AP or 3 AP/2 AD champions
Against a well balanced team there are MULTIPLE factors that come into play that are hard to really throw an item list at and say, "GET THIS!" Are there a lot of melee champs? Are they nukes, or sustained damage? Look at how they've built themselves and react accordingly.
*Warmog's is ALWAYS nice.
I won't say it again. Buy these every CHANCE you get. They're completely invaluable and as a "Support Tank" people will think you later for it. These items win games, not
Rabadon's Deathcap or
Force of Nature... a simple little 75 gold item wins you the game!
Pros / Cons
Pros:
Her passive(nuff said)
Her crowd control
SHE HAS THE SUN ON HER SIDE
Cons:
If you don't have any synergy with your lane mate, you will look foolish. Initiating at the wrong times can and WILL lead to your death with your carry left to defend them self screaming, "WHY LEONA?!" so make sure you play Leona in a lane with someone you know and can work well with.
Laning and Creep Killing
Don't even THINK about hitting minions in lane unless you are by yourself and hugging your tower. Don't take away from the possible gold that your carry could be getting. If you sit around and blast all of the minions with your Eclipse and auto attack the minions to death then you deserve your loss and that's all I'm going to say about it.
Summary
If you have any questions, comments or concerns then leave them on the guide page and I'll be sure to get back to you with some sort of response. I'm publishing this guide just to get it out there as there doesn't really seem to be any decent Leona guides on this site. I'll be doing some tweaking to the guide over time to make it look "prettier" and more user friendly and I'll be adding new sections later such as "Ranked Play" and "Team Work" to give you all some helpful tips on how to use her ultimate and in what situations you should use 
Zenith Blade.
Thank you for taking the time to read my guide and I hope you find it helpful. See you on the battlefield summoners! =D
Build Guide
Discussion (18)
Greater Mark of Insight runes to help add that extra kick to her abilities and supplement her lack of damage. The 
Greater Seal of Resilience and 
Greater Glyph of Warding to give her that extra tankiness early game, because who needs MagRes per level when you have a spell that supercharges both armor AND magic resist?
Greater Quintessence of Swiftness for one reason and one reason only and that is to help lane your 
Greater Quintessence of Fortitude don't really seem like they're worth it to me, but then again this is all about personal play style. I've used the health quints and though I do like the extra starting health that it gives you I found that it can be quite hard to catch up to people and land that 
