My paladins:
Nahyomi: level 60 party support build (31/11/9) with BoK and uber healing skills. She's able to tank undead instances, much to my surprise but she's not a tanking build. She's what I call a utility infielder build -- in most instance runs, she does moderate damage, takes moderate damage and does moderate healing. In healing gear, she can heal any 5-man instance in the game, and has solo-healed Baron runs more than once. She does require two full sets of gear to function at her highest capacities.
Malka: level 42, on her way to a stronger healing build than Nah. I still want BoK for her, for party support, but I think she will be 40/11/0. I don't expect her to be able to tank at all. If I decide this is boring I will switch her to a reckoning build (21/25/5 most likely)
Charys: level 45, on her way to a critidin build of 21/0/30. This build should also be able to tank undead instances and demon bosses, but not others. Maybe OT in Tier 0 instances. Maybe.
If I were building the strongest tanking build I could, I'd be remembering that threat for a paladin comes from doing damage, period. At the same time, a tank needs to be able to take damage, so will be fighting 1h/shield. I don't think you can tank without consecration, so you'd need 11 points in Holy.
I'd try
this specc I think.
It's a hard call not to take Holy Shield, but I was trying to maximize damage dealt. It might be true that replacing the 5 points in Reckoning with 5 points in Imp Blessing of Might (from the ret tree) would have a bigger payoff. I'm not sure.
This is a specc that can't heal worth a darn and mana will be low, too. Even the Holy talents were chosen to increase fighting damage, period. You'll be in 1h/shield, so you will use SoR. Even though judgment and SotCr were not improved in this talent specc, you'd still want JotCr at the start of each fight to up the holy damage you do with SoR, JoR, and Consecration.
Everything else is chosen to incrrease threat (by increasing damage) or increase the ability to take damage.
Now, I haven't tried this specc myself, but if I wanted to be the best tank I could be, this would be my first shot.
I'd equip this paladin with a mix of gear, including +crit gear (to increase damage done), +attack power, and +stamina gear, as well as some int (probably from Sacred Protector and jewelry, mostly) to get enough mana to keep JoR and SoR going for the entire fight. My first choice blue high-50s weapon would be Serathil, because of the armor bonus, with either agility (for armor and dodge) or +9 damage on it. If I were really hard up for int, I might put +22 int on that axe, and use jewelry, shield, and trinket slots for other stats. Find a friend who can make it, and put a Cloak of Warding on this palli.
I do not think this is a great leveling specc, however. For leveling, I suggest putting the first 21 points in Holy to get Consecration and Divine Favor, and then climbing the ret tree. (Divine Favor helps you be self-sufficient because it gives you a crit heal that costs zero mana once every two minutes -- big giant win for a soloing or duoing paladin.)
The tanking specc I outlined above is going to be the slowest killer ever. And paladins already kill slowly! When you are tanking in a group you don't care because as a tank, killing the enemy is not your job: holding its aggro is your job. However, leveling solo or with one or two people on outdoor quests is going to be painfully slow with a tanking specced palli.
Your approach of putting points in ret first can also create a fun leveling build, although I'd put the next 6 points in Holy to get to consecration if I were you. Without Divine Favor, you are not going to be able to heal well, but that really doesn't have to matter.
I'd switch to the tanking specc around level 55, I think.