Sona

As Sona you want to poke whenever you have cd on q. With Leona you have to just stay back first three levels, then try to fight sona with full comb. Sona with just e movement spell is easy target to catch for you.

Soraka

Soraka does not have a good movement at all, if you catch her with your e w q and use ignite, it should to be free kill.

Blitzcrank

Whenever blitz hook you, you can just stun him, or his ad carry.

Alistar

Alistar can easily interrupt your e with q w, but then he does not have any cooldowns to deal damage to your adc, this is moment when you try to poke their ad carry the most.

Lulu

Lulu is quit good counter pick to Leona. Just because of sheep and nonstop shielding ad carry. Try to evade poke from her q´s

Thresh

Thresh can be good counter pick for Leona, but... when you use your e, he will interrupt you with his e, you still can stun him with q.

Morgana

When Morgana use shield at herself, you stun ad carry. When she shields ad carry, stun morgana

Bard

He will try to poke, just full comb him then he will cry for whole game.

Zyra

Good poke, even if you catch her she will do a lot of dmg with plants, its hard to play vs her.

Nami

Nami with movement, healing, good damage abilities and cc's on Q and R is best counter to Leona. If you play aggressive she will out traded you, but when you play defensive she will destroyed you.

Rakan

I have just few games played versus Rakan, but i can say surely he is quit op against Leona because of his mobility a cc's.

Nautilus

Nautilus is the same play type like Leona, but he has extra shield on w. Try to avoid him and go for ad carry.

Zilean

Zilean is for me the most annoying champion, he will bombing you and your ad carry for whole game. Even when you take a good attempt and kill his ad carry, he will just used his ult to keep adc alive.

Ashe

Caitlyn

Draven

Ezreal

Jhin

Miss Fortune

Tristana

Vayne

Xayah

Lucian

Janna

Jinx

Kalista