Well guys first off all I have to say that this is my first build so I dont know what can i say to you, and my english is not the best but I try to wrote it the best I can.

Leona looks like a cross between Jarvan and Amumu. She has the same ability to "pull" herself into the enemies, like jarvan's Flag+Spear combo and Amumu's bandage. She also has single target CC, like Amumu. Then, she has a shield-like defensive spell, like jarvan's actual shield. That spell will also do some AoE damage, like Amumu's tantrum. She also has a large-radius AoE CC spell that can essentially allow her to overcommit to a kill as a tank, like Amumu's Ulti, although not as powerful CC wise.
The interesting thing is that Leona basically NEEDS her team to do good damage and be effective. I would recommend only playing her at an elo or with a team that can follow up to your movements to initiate and counter initate. A tank should be the engager, and Leona is very well suited for that role, just like Jarvan and Amumu are.
I think that Jarvan has a better initiation of the three. Amumu has the better counter-initiation. Leona is in between as she won't be able to cut an entire group from the rest of their team and can't actually HOLD anyone in place. She only has her Q for stun and her ultimate if you are good enough to land it directly on someone. Through that, I think that she will do the most damage out of all of them with her passive Sunlight. She is potentially able to get 3 stacks of sunlgiht on multiple people right of the bat, if timed right, and 4 stacks on 1 person with the addition of her Q.
I think her ultimate won't be as gamechanging as Jarvan or Amumu's but will still have much of an impact on a team fight as it will supply Sunlight stacks and will do lots of damage and slow, as long as you aim it right. The stun won't be absolutely necessary, but it can be beneficial. Placement of the ult to capture as many people as possible should be the first priority over the stun.
I think that Leona will be played passively to farm if you have a solo lane and agressively to deny and harass in a duo lane. I don't think that solo lane suits Leona well because of her passive and how she needs an ALLY to attack the target with sunlight for the damage proc. If you get a solo lane, you are missing out on quite a bit of damage. I don't know how this might work though. Only actual experience will tell.
Overall, what I think will seperate the good Leona's from the bad is the ability to land the stun on the ultimate and the ability to time your spells just right to get the maximum effect from Sunlight.

Max ultimate first, as always.
Max Eclipse next. The extra damage and resistances will make harassing the enemy a LOT more effective. You will do much more damage to them while they will do much less damage to you.
Max Zenith Blade next to maximize your AoE/multi-target damage. As a tank, that will make you a much greater threat.
Max Shield of Daybreak last. The lower cd and increased damage won't compare much do the AoE damage of the other spells when you are a tank.


User preference though, as always. If this skill set-up doesn't suit you, try a different one.

Build like jarvan in the beggining, as you will need the items later on, and will need to farm/harass effectively. Philosopher stone and HoG should be one of your first item goals.
Boots are obvious.
Then, build sunfire cape for the AoE DoT. You'll become more of a late-fight threat, like amumu, while gaining tanky stats.
Rylai's Crystal Scepter Helps give you an AoE slow, which helps you set up for your engagements and counter-engagements, while supplying you some AP and HP.
The rest of the item build is situational and item preference.

Don't Underestimate CDR on Leona. Just CDR essentially gives her more CC, more positioning power, and Durability. Don't build CDR right away. Build your initial tank items first, or else you won't be able to make good use of your CDR.
CDR items: Frozen Heart Randiun's Omen Shurelya's Reverie.

Well, if you like my build Vote for it and if not please explain me why or what is missing in this Build because this is my first build as I say before. :D
Sorry me if the guidl don't have the images of items and skill, I don't know how to put them.