With leona, she really doesnt do enough damage as a pure tank to be better than any other tanks, but tanky dps, she can tank decently enough while dishing out decent dmg and cc. With this build, you can pretty much buy the items in any order needed after the triforce. However, know that leona doesnt really fit any role better than most other champs, and so is more of a fun cc champ than anything else.

All the items after tri force are pretty optional. If u want, you can upgrade that heart of gold into a randuin's omen. U have a lot of health, so an atmas isnt too bad. If u like ga, or you are getting focused and cant handle it, GA is always useful, but i like health on her more since u get a free 60 armor and mag resist.