(This post has been updated a few times since it was first published. Gear has been added as it was discovered in harder loot tables. My gearing perspective evolved as I got experience with 25-man Ulduar which affected the items I included. More updates may be needed if more gear is discovered as the world sees more Algalon kills, but otherwise these lists should be stable. — Z, late June, ‘09)
First, thanks and credit for inspiring this post are due to Siha’s best-in-slot post for the Naxxramas era of Wrath. I found it when I first speced holy and it was a great resource.
Let’s review what “Best-in-slot” means for Holy Paladins. The notion of BiS derives from a simple model for which you can chose mathematically optimal pieces for each slot. This works great for optimizing a DPS rotation which has to burn down a giant health bar. It doesn’t work for coordinating a group of healers who have to avoid over-healing 25 little health bars. Whether or not a drop is an upgrade depends on your current gear and your raiding environment. Are you dropping Holy Light bombs on a few targets? Are you spraying Flash of Light around the raid? Is your guild aggressively stacking and putting you in some specific utility role? Are your elemental shammies and squishies passing on that fantastically itemized non-plate drop?
This post doesn’t list the best pieces that every holy paladin should have because each paladin will have different answers to those questions. This is a list of the pieces that holy paladins will have on their radar while making their way through Ulduar.
To keep the list reasonably small I have to leave some things out. This list only includes drops found in Ulduar. I’m only including exceptional non-plate items which don’t have comparable plate pieces. I’m not including any items with hit or spirit. I’m continuing my proud tradition of ignoring PvP entirely.
If you want to see the raw lists that I work from, including the above omissions, you can go straight to the source. I use a wowhead search with a weight scale that normalizes holy pally stats based the amount of each that you see in gems of similar quality. This ends up listing items sorted by the amount of item budget allocated to stats that holy paladins care about. It’s up to you to decide which of those stats are relatively more important to you.
I’ll tend to order the items in each list based on the gearing style of stacking int and crit while maintaining a comfortable level of haste. Again, upgrades are evaluated in the context of your raid. There is no universal measure of relative upgrade value and my ordering should not be taken as such.
Finally, I thought it would be fun to try visualizing the different properties of the items in the list. We’ll use icons to indicate the armor level of items as well as which of the holy pally trinity (crit, haste, mp5) the items have, as follows:
Plate armor items
Mail armor items
Cloth armor items
Items with crit and haste, and thus no stinky MP5
Items with haste, MP5, but no crit
Items with crit, MP5, but no haste
Here we go!
Head
[Conqueror's Aegis Headpiece] Thorim 25, 58 
[Ancient Iron Heaume] Iron Council 25
[Valorous Aegis Headpiece] Mimiron 10
[Faceguard of the Eyeless Horror] Yogg-Saron 10
Our head upgrades are well itemized and relatively easy to get. The tier head piece is an excellent use of emblems if you’re having trouble with squishy classes grabbing the conqueror tokens.
Neck
[Pendant of the Shallow Grave] Thorim 10H
[Charm of Meticulous Timing] XT 25H
[Pendant of the Somber Witness] Algalon 10
[Freya's Choker of Warding] Freya 25
[Frozen Tear of Elune] 19 
[Pendant of Endless Despair] General 10
Our neck upgrades are all over the map. Most of us won’t see at least half of them. Those that remain aren’t that compelling when compared to [Cosmic Lights]. The pendant of the shallow grave just screams “holy paladin” — expect your pally healers to lobby hard for attempts at Thorim’s hard mode.
Shoulder
[Pauldrons of the Combatant] Thorim 25H
[Conqueror's Aegis Spaulders] Yogg-Saron 25
[Razorscale Shoulderguards] Razorscale 25
[Valorous Aegis Spaulders] Thorim 10
[Pauldrons of Tempered Will] Ignis 10
We have strong options in the shoulder slot. The tier pieces are well itemized, though prying the 25-man token from Yogg’s many-tentacled grasp will not be easy. The pauldrons of tempered will are easy to get, have nice stats, and their generous stamina tries to make up for the lack of sockets.
Back
[Drape of the Sullen Goddess] Freya 25H
[Shroud of Alteration] Trash 25
[Shawl of the Caretaker] Ignis 10
[Drape of the Spellweaver] Trash 10
[Cloak of the Dormant Blaze] Trash 10
Our cloak upgrades are such a mess that it’s a stretch to call them upgrades. One 25-man cloak outside of hard mode, and it has haste and MP5? The only cloak without MP5 drops from trash and is arguably a side-grade from the [Shroud of Luminosity], never mind the [Pennant Cloak]. Grats, casters, holy pallies will be passing on your cloaks in Ulduar.
Chest
[Breastplate of the Devoted] XT 25H
[Conqueror's Aegis Tunic] Hodir 25, 58 
[Chestguard of the Fallen God] Yogg-Saron 25
[Breastplate of the Stoneshaper] XT 10H
[Valorous Aegis Tunic] Yogg-Saron 10
[Breastplate of the Afterlife] Razorscale 10
Let’s take a moment to appreciate the breastplate of the devoted. Most of us won’t see it, of course. The remaining significant pieces without MP5 aren’t exactly trivial to get to. Most of us will be throwing up our hands and just spending the emblems on the T8.5 piece.
Wrist
[Bindings of Winter Gale] Hodir 25H
[Horologist's Wristguards] XT 25
[Unfaltering Armguards] Kologarn 25
[Bracers of Righteous Reformation] Trash 10
[Armbands of the Construct] XT 10
I like the symmetry of our wrist upgrades. Do you want haste or crit, from 10- or 25-man? One of each is on offer, though getting a trash drop will be a roll of the dice. The cloth bindings are just ridiculous. Be prepared to make a squishy enemy for life if you try to nab a pair.
Hands
[Pharos Gloves] Algalon 25
[Gauntlets of the Thunder God] Thorim 10H
[Conqueror's Aegis Gloves] Mimiron 25, Emalon 25
[Gloves of the Pythonic Guardian] Kologarn 25
[Gauntlets of Serene Blessing] 28 
[Valorous Aegis Gloves] Emalon 10, Freya 10
Thorim’s hard mode just got that much more interesting, good grief. The rest are a few paces behind and don’t compare all that favourably with the strong itemization of T7. At least the pythonic guardian gloves have some crit.
Waist
[Belt of the Fallen Wyrm] Razorscale 25
[Plate Girdle of Righteousness] Blacksmithing BoE
[Girdle of Unyielding Trust] 28 
[Belt of Clinging Hope] General 25
[Cable of the Metrognome] Mimiron 10
Our waist upgrades are another sorry lot. If you see holy paladins with goofy blue belts against their pretty red tier dresses, you now know why. Many might well stick with their [Waistguard of Divine Grace]. The slots in the plate girdle will be nice, if you can afford it.
Legs
[Conqueror's Aegis Greaves] Emalon 25, Freya 25
[Frostplate Greaves] Hodir 25
[Legguards of the Peaceful Covenant] 39 
[Valorous Aegis Greaves] Emalon 10, Hodir 10
[Legplates of Flourishing Resolve] Freya 10
Our leg upgrades give us another opportunity to tweak our crit and haste balance with relatively easy to get pieces.
Feet
[Treads of the False Oracle] Yogg-Saron 25H
[Greaves of the Rockmender] Auriaya 25
[Treads of Destiny] Blacksmithing BoE
[Sabatons of the Iron Watcher] Kologarn 25
The stats on those treads are sufficiently amazing to endure the teasing that comes from healing in slippers. The greaves have crit and a nice socket which make them popular, but otherwise this slot can be used to balance haste and crit as needed.
Finger
[Nebula Band] Algalon 10
[Starshine Circle] Algalon Quest 25
[Pyrelight Circle] Ignis 25
[Fire Orchid Signet] Freya 10H
[Starshine Signet] Algalon Quest 10
[Ring of the Faithful Servant] Auriaya 25
[Sanity's Bond] Yogg-Saron 25
[Radiant Seal] Iron Council 25
Our ring upgrades are a little bonkers. Are there really no rings without hit or spirit in 10-man before hard mode freya or — seriously — Algalon? That can’t be right. The excellent Pyrelight Circle makes up for this by dropping from Ignis, which even pugs will have no trouble with. Expect to see lots of paladins who aren’t thrilled by MP5 hanging on to their [Signet of Manifested Pain] or [Inscribed Signet of the Kirin Tor].
Trinket
- [Pandora's Plea] Mimiron 25
- [Eye of the Broodmother] Razorscale 10
- [Meteorite Crystal] Algalon 10
- [Scale of Fates] Thorim 25
- [Sif's Remembrance] Thorim 10
- [Energy Siphon] Flame Leviathan 10
One hopes to collect most of these trinkets over time so that they can be swapped in and out to match the needs of specific fights. The eye of the broodmother deserves specific mention for being a great trinket that is available early in the Ulduar progression.
Main Hand
[Val'anyr, Hammer of Ancient Kings] 30
+ Yogg-Saron 25H
[Aesuga, Hand of the Ardent Champion] General 10H
[Constellus] Hodir 25H
[Guiding Star] Razorscale 25
[Pulse Baton] Mimiron 10
The excellence of Val’anyr and Aesuga are matched only by how difficult they are to get. Constellus is no slouch, but is also hiding off in hard mode. Many of us won’t exactly be lining up for the remaining accessible upgrades in Ulduar if it means losing crit from our [The Turning Tide] or even [Hammer of the Astral Plane] or [Life and Death]. But hey, if nothing else, we can all agree that it’s cool to see sockets weapons.
Off Hand (shield)
[Wisdom's Hold] Thorim 25H
[Ice Layered Barrier] Hodir 10H
[Pulsing Spellshield] XT 10
At least the small number of caster shields simplifies our lives. the pulsing spellshield is a decent and very accessible upgrade for those of us who weren’t lucky enough to pick up [Voice of Reason]. If you were, however, you’re sitting on your hands until your guild can manage hard mode 25-man Thorim. Leave the ice layered barrier for our healing shammy colleagues.
Libram
- [Libram of the Resolute] Auriaya 25
And finally, an uninspired 15% upgrade from [Libram of Tolerance] to add to our collection. In fights where we can heal from melee range and receive near-infinite mana from Seal of Wisdom we can swap this in for our trusty old [Libram of Renewal].