Leona

As a bruiser, Leona is a more viable choice. I haven't played her enough to know if she can compete with "the big three" (Warwick/Irelia/Udyr) but right now, she seems pretty solid as the top lane choice.

As everyone is saying, she's way too squishy and easily crowd controlled to be a competitive main tank pick but as a bruiser, she's certainly able to do a decent job. Right now, she feels *a lot* like Poppy and to be honest, I see her being thrown on the same pile of "niche" trash champions like Poppy unless she gets a bit of TLC. Riot seemed pretty proud of their first female tank but in all honesty, she falls pretty flat if played as a tank. Seemingly, she has the highest base armour and second highest base health but somehow, she just doesn't feel like it at all.

Still, give her time and she might end up becoming a decent bruiser pick - that's how I've been playing her and it's certainly paying off right now.