Though I haven't played any Darksiders games, I've taken particular interest in a few items. Such items would include the Rod of Arafel -- including the Eye and Staff of Arafel as well -- and also the Voidwalker. Doing Wiki work on DD Wiki made me highly interested in all of these items. The search for the Rod and it's scattered pieces intrigued me greatly, while the Voidwalker on the other hand, heavily reminded me of the Portal Gun from the Portal series.
I'm not sure if it qualifies as an item, but I was very fond of the Abyssal Armour from the first game. It was fun to collect, it really did give a helping hand (especially on Apocalyptic) and it looks awesome. What's more, the ability to transfer the item when you started a new game is a great idea and gives many players playing on normal or easy the incentive to play through again on the harder difficulties. I just felt that the item was rewarding and well worth collecting all the pieces. Also, unlike Darksiders II the items aren't cheap -meaning you don't get a new one every 4 and a half seconds- so that made this item all the more awesome.
If I had to pick a favourite item from Darksiders II though it would have to be any of the possessed scythes. I thought this was a really cool idea and what's more it gives purpose to the endless loot that you pick up other then just exchanging them for money as you can pick the attributes you want your possessed scythes to have and use those items to upgrade the scythes. The Chaos Fang was pretty cool too.
Well considering almost every item in DS 1 and 2 is a direct Zelda derivative, you could ask this question in another section that shall not be named of this site, but there is one notable exception - the Soul Splitter. Sure, everything is derivative in some way, even the Soul Splitter, but I think it is utilized in some of the most clever situations in the entire series, and stands out with not just splitting yourself, but leaving the statue behind that is where your physical form always has to stay (and sometimes figure out how to move), creating some very cool puzzles.
And, it is hilarious that it breaks many of the combat encounters in the game. Lead to some good laughs throughout the many, many hours I spent in DS2.