For serious, by-the-book survival and CC with a little utility, Abyssal Malletfire is definitely my better guide. For having a bit of fun and leaning a bit more toward damage, read on. Abyssal Malletfire is also more thorough and most parts of it are written more recently, so as an intro to how to play Leona in general, it is much better. This build leans toward 3v3 and is more about having a little fun mixing it up a bit without going straight to trollsville. This build is also slightly more appropriate for solo top, leaning toward tanky DPS more than I'd usually advise with this champ. To be honest, I play supporty tank when I want to win, but if you want to experiment, you can try this out.

This AP tank build emphasizes maximizing Sunlight damage. Since Sunlight doesn't scale with AP or AD, the only thing you can do is lower MRes. Not worth the trouble strictly for its own sake, but if you have enough magic damage on your team to justify it, then using MRes reduction for your passive and your combos is a nice bonus indeed.

This build is attempting to be something of a Nasus for mage-heavy teams. Where Nasus is an AD tank that lowers armor with Spirit Fire (synergizing Black Cleaver), this particular Leona build takes analogous MRes-reducing items and splashes Sunlight around (instead of lowering MRes even further with something like Spirit Fire).

This build might be better for 3v3. It's not the kind of thing you want to play alongside 4 carries. I think it's fun in 5v5 as long as you at least have an additional tanky DPS on the side.

* I took out Philosopher's Stone under the assumption that you will be solo top. Rejuvenation Bead is taken but sold at the end; Warden's Mail/ Randuin's Omen can potentially be taken, ultimately replacing Phage.
* Nashor's Tooth is taken slightly earlier than before.

For some reason, everyone in the forums thinks that only noobs and trolls build AP on Leona.

Not really. At least for now, her AP scaling isn't that bad:

* All 4 skills (but not her passive) scale with AP
* 3 out of 4 are multi-target attacks
* It's very much possible to combo all 4 skills onto a single target: WEQR.
* Her AP ratios sum to 1.9. Sure, .4 on E and .4 on Q are small individually, but you'll be using them back-to-back a *lot* and following with another .3 from W.

Not the best in the game overall, but you definitely *won't* get ignored if you pop WEQR on their carry with 150 AP and some MRes reduction. Honestly feels pretty powerful to me. Sure, you can't 1v1 after your burst, but you're not supposed to go in alone. You're still the tank.

Meki Pendant and 2x Health Potion for a first trip to lane. Builds into Fiendish Codex early-ish in the game, as CDR is too good to put off for too long.

Sorcerer's Shoes is taken here as a bold departure from typical playing. Mercury's Treads is still an option, but if you want to experiment with DPS Leona, then that's a compromise.

Sunfire Aegis is a nice item for making them pay a little for ignoring you.

From here, the philosophy differs from what you might be used to.

From now on, I'm taking debuffing effects. First I take Malady. This works with Sunfire Aegis, Eclipse, and so on. Abyssal Mask gives some more nuke power, MR, and additional MR reduction; you might take this before Malady.

Nashor's Tooth comes next, giving great CDR and AS to synergize with Malady.

Finally, I simply listed Phage as a last item, because Frozen Mallet and Trinity Force are both good options. You could even go for Warmog's Armor, Randuin's Omen, or Lich Bane according to taste. Trinity Force or Lich Bane are squishy but make the build do hybrid damage, Randuin's Omen or Warmog's Armor make this build fairly tanky (with magic damage), and Frozen Mallet is not quite as tanky but has more utility.

Up to you I guess. HP5 is a bit lacking in the itemization, so HP5/level seals are an option.

CDR won't quite be capped with Nashor's Tooth + masteries, so this should be fixed.

MPen marks stand out, and MR/level glyphs too. Move quints are always appealing.

Exhaust + a spell of your choice. Exhaust is just too good to pass up, allowing you to keep an enemy in deep trouble for a long time, maybe even long enough to combo them a second time at the end.

Flash is well-established as a great spell. Can be used in place of Zenith Blade, for example. Or you can combo at low health and Flash out if you have to. Whatever.

1/28/1 on the new tree. Love the 8% CDR on the defense tree and the gold per kill/assist.

First of all, never solo lane. Your passive is too good to waste.

Farm with Eclipse, mostly. Zenith Blade on the side if you want, but it's finicky and usually not worth it, and can pull you toward champs if you're sloppy while not doing much damage.

Note that you can use Eclipse to last-hit, or you can use it on healthy creeps to help a lane mate farm (weakening them and applying your passive). Or you can do something in between, killing a couple and weakening a few more. In any case, the total amount of farming done by your lane should be large.

It's annoying when you're stuck with a lane-mate who doesn't (seem to) understand what Leona's passive does. In that case, either explain if you have a moment, or just forget about it.

There's a bit of cost-benefit analysis involved in deciding how much you really want to use Sunlight in the lane. If your WEQ all fire back-to-back, your lane mate might only get one Sunlight proc. On the other hand, if you don't CC your enemy, they might just get away. Ideally, you should be able to alternate hits, and maybe your lane mate can CC in between your stuns and you can use Sunlight to the fullest. It would be nice. But do whatever works, and keep in mind that under some circumstances, you can afford to wait a second on your next skill. Exhaust instead of Shield of Daybreak, for example.

In large fights, often damage is being dealt so rapidly that you couldn't combo so fast as to waste Sunlight anyway, so that makes the choice easy. Just combo away at that point.

Singed, for example, has an easy time consuming marks with poison damage, and a Miss Fortune ult easily could eat a combo's worth of marks.

Zilean is pretty fun. If he puts a Time Bomb on you around the time you start Eclipse, then by all means, deliver the package to its intended recipient with Zenith Blade. Stun lock or Exhaust or whatever to stay in range if there's extra time on the fuse.

Pros:

Tanky enough
MRes debuff(s)
Slows
Stuns
More slows
Even more slows
Nasty anti-carry combo, WEQR
More DPS than you might think
Ideal for magic-heavy team compositions (main build)
Ideal for quickly focusing one enemy (single-target 44 MRes reduction overall, main build)
Appropriate for solo top

Cons:

Could be tankier still (alternate build is closer to max tankiness)
Inappropriate build for AD-heavy teams
Teammates might flame you for taking ASpd
Not appropriate for 0 cs support play

Leona's default portrait looks like my ex-girlfriend. True story.