So I've been playing quite a bit of Leona lately and I wanted some input on my build with her. I play her as a tank of course that exists to support the carries with max cdr. However, I've been building Trinity Force as well because it seems to greatly increase my damage, especially vs the squishier targets.

Here's the build:

Start with 2 ruby gems(sometimes switch one for dorans shield), boots, and 2 health pots
Get glacial shroud and kindlegem ASAP, this brings me to 25% cdr, combined with masteries and runes brings me right to 40% cap when i upgrade to Frozen heart.

Next is Merc boots followed by either phage or sheen if I'm surviving enough or wardens mail if I need more defense.

Trinity force is next and since I am spamming her spells so much I get many procs off of it.

Final build is:

Merc Boots
Frozen Heart
Randuins Omen
Trinity Force
Kindlegem ( upgrade if necessary depending on game)

Last slot flexible based on needs, Odyns Veil commonly.

Any input on whether or not this is a solid build? I seem to be doing pretty well with it but am always looking for improvements.

Also, what marks should I be running? I keep switching between ad and ap pen, same for the masteries, I can't decide between 10% arpen or 10% ap pen. Thoughts?

I like double ruby, boots of correctness, aegis, glacial then evaluate. This gives you a decent smattering of all defences + some cdr and at this point in the game you have a feel for who is going to be the biggest threat and if its time to build dps yet. Almost always grab phage -> trinity force at this point, but often finish off frozen heart and/or grab a negatron cape before the zeal -> final build of trinity. Phage and sheen are both really decent on their own for her, so not too big a loss doing it like that.
I'm not sure about runes / masteries. I suspect since her spell damage is so low and I rely on trinity force or wits end for what I jokingly refer to as my "dps" that I'd be better served with Arpen runes than the Mpen ones I've run so far.

Two big ticket Armor items, but no MR aside from the merc treads until after both those AND a TF? It's true that armor is more often the primary damage type, but it's rarely wise to build only one defense.
I would put an Abyssal Scepter or a Force of Nature in place of Randuin's. The scepter gives you and your team more damage while FoN boasts much more MR, Health regen, and some nice bonus movespeed. I tend to go AS if there mages aren't bothering me much and FoN if they're being problematic.

Going for CDR is the right idea, and I'm trying to work TF into my build as well but so far don't have a solid slot for it. Even if I can't finish it, the Sheen proc is nice on her Q and Phage is always useful.

We start off similar, but instead of two gems I just build the Kindlegem right away, boots, and one of each pot. I'm rarely low on health in the first fight, either dead or doing fine. That's why I take the mana pot, because if the fight goes on for a long while I find myself short on mana.

Every now and then I think about how maybe I should use FH instead of Randuin's Omen as my main Armor item, but ultimately I stick with Randuin's. I reason that, as much as I want the CDR, Randuin's gives me health. I don't like only having a kindlegem/upgrade as my single Health item, considering that I only buy Phage late game.
And lets not forget Randuin's whopping 5% CDR! 8D

As far as runes/masteries, I go 9/21/0 with Mpen in the Offensive tree. I figure the pre-requisite CDR mastery and the mpen together are worth more to me than the bonus AS and ArPen of the other masteries. Likewise, Mpen marks, but tanky everything else.

As far as rating your build... I'm bad at throwing down solid numbers like that. I think there are ups and downs of both our builds, best I can do is to show mine for comparison and tweaking.

This is the general idea of what I do.
I start boots, 2 dorans and 2 health pots.
I usually grab glacial shroud then upgrade boots to Tabi or Mercs based on matchup.
I try to grab some MR at this point. Spirit Visage is nice(especially with the Dorans shields). Though Ive also gone Aegis if playing with squishies or just grabbing a negatron to be upgraded. Dont overlook Hexdrinker here, especially against bursty casters. Plus, due to Leonas ability to survive at low health, Maw can be useful.
I usually grab sheen first. The only problem with sheen is that comboing stuns wastes procs...you can avoid it but your CC is more important than damage. After this ill go phage then finish trinity.
I've also byassed trinity entirely and gone for witts end

I'd get a MR item before turning your wardens into Randuins.

Also, as a personal preference I like to take the mpen mastery mainly for the 4% CDR, which lets me rune for MR on blues.

If you want armour and CDR I would just, instead of finishing Randuin's, build Glacial Shroud and Warden's Mail. It's cheaper too.

She is a tankie champ and most prefer Alister since he adds just as much or more.

what exactly does alistar add that leona doesnt? alistar is a pure tank with very little cc. he has a headbutt and a knockup. and usually people use them together. he's a glorified malph ult with a shorter cooldown. theres absolutely no reason to attack an alistar. he has no taunt and no dmg. the most he is, is annoying.

leona is better because she has cc. she has a reason to be attacked. if u dont attack her, she'll keep stunning you and shutting u down, as well as boosting the dmg of her team. it might be small but it adds up after a while.

I find that Alistar is much better at defending a tower but Leona to me seems better in small skirmishes or chases where she excels at locking down people. She also contributes more to team fights with her passive, I can easily hit a few people with Eclipse or her ult and then lock down the carries with her Q and E.

I like your build, as i go similar but I get tabi instead of mercs and get aegis and instead of randuins I play it by ear with other armor builds.

It took me a while to get into building Trinity but after those many times of being the last player alive trying to kill 3 enemies with super low health, I learned it pays off.