EDIT EDIT EDIT:
I'm still getting hits from search engines regarding DK trinkets, so if that's what you're interested in, proceed to the bottom of the page for a quick list on DK trinkets.
This post was originally made after DBW (normal) dropped for me in ICC25.
I'm tempted to call it the best item ever.... but the title surprises even me, because I lusted after the Dragonspine Trophy from silly old Gruul's Lair for soooo long (and never got the damn thing after scores of kills). I think I'd probably still have a more powerful response to a legendary weapon like Shadowmourne, but it's close.
Yes, I have to say Deathbringer's Will is a smash hit! (get it?)
It certainly fits the bill for best dps trinket of the Lich King expansion, providing a healthy dose of armor pen all the time - which scales extremely well in full ICC gear, but it also has the most enjoyable proc I've experienced yet in the game. The proc itself happens often, giving an amazing (in my DK's case) 600 haste, crit, or strength, and it lasts a full 30 seconds.
Best of all, it actually transforms you into a Northrend race for the duration of the proc!
For haste, you get to be a Vry'kul (a little undersized imo).
For crit, you become an Iron Dwarf
For strength, you'll be looking at a fearsome Taunka!
It's a lovely, and enjoyable thing.
The only problem with the trinket that I hope Blizzard fixes soon is that whenever the proc occurs, you can't use mounts and some other actions become impossible. I can understand if Blizzard doesn't want me flying as a Vry'kul (sort of), but in instances it becomes a somewhat significant issue. For example, on Gunship, you can't use the rocket pack while the proc is active, so if you want to fly to the enemy's ship, or back again, you have to click off the proc (losing the buff and the cool animation) before you can fly.
In any case, I've still been having tremendous fun with it, and I look forward to my Beauty and the Beast proc for months to come!
EDIT EDIT EDIT
Given that google searches tend to dump people here when looking for the best DK trinkets in Tier 10 content, here's my list (varies only slightly from what EJ says)
1. heroic Deathbringer's Will (I was fortunate to get this upgrade last week)
2. heroic Sharpened Twilight Scale
3. Deathbringer's Will
4. heroic Death's Verdict
5. Sharpened Twilight Scale
6. Herkumi War Token
7. heroic Whispering Fanged Skull
8. Death's Verdict
I arrived at this list by assigning values for each stat (attack power equivalence) based on several sims I use - similar to how Elitist Jerks does it. Then, you just calculate trinket uptime, arrive at a final AEP for each trinket, and voila!
Hope that helps for those of you looking for something like this and not just my gushing over a trinket I got months ago. :)