In terms of money value?
For me it was a sanguine longbow which I sold for 1.2 mil.
Next up were several lilith and orlok items (shield, wand, bracer).

When talking useful though, its the shivering set.
When I ran my bird up in levels I noticed that no other item really gave better damage/protection etc. then a full shivering set until reaching lvl 45. Got a lot of mileage out of that stuff all the way from lvl 30...