Interesting topic. Really made me stop and examine what jewelry I use and why.
I organize each character's stash like this:
I'm a big fan of +All Skils jewelry. I guess it's a holdover from playing Diablo 2, where it was a rare, godly modifier, and Stones of Jordan were currency for trading. In terms of what my Sacred 2 characters actually wear in their ring/amulet slots, it's mostly blue/yellow jewelry with +All Skills and whatever best additional modifiers I can find (listed below).
I collect -% Regen Penalty from Buffs amulets because it's a great alternative to using Buff Suits (which are a headache). I keep a weapon and shield with high number of sockets in the far right weapon slot (usually Rancuil's Dark Flame or Excalibran, which have this modifier natively), and pack them full of -% Regen Penalty from Buffs amulets. I use these weapons only for casting buffs.
"Useful Rings and Amulets" are mostly blues at lower levels, and uniques & yellows at higher levels. The modifiers these pieces have that make them valuable to me are:
Defensive:
Damage over Time: All Types -X% Very difficult to protect against this, and in Ice and Blood it can take huge chunks out of your hitpoints. Very important. Stacks with diminishing returns, it will never reach 100%
Opponent's Chance to Wound/Weaken/Burn,etc. -X% The secondary effects can be bad, particularly Freeze and curses that Weaken. Stacks with diminishing returns, it will never reach 100%
Damage Mitigation: All Types +X% Very powerful against flat damage from all attacks. Stacks without diminishing returns, allowing for 100%. This is OP, and due to a bug, but good news for the player, as it can make them invincible to most attacks.
All Combat Arts +X or Aspect +X Full upgrade to Combat Arts with only half the penalty. I value these because I never socket runes (probably should, just don't).
Regeneration Time -X% this is always nice but as there are many ways to lower regen time it's not a priority with jewelry
Max Hitpoints +X more hitpoints, live longer
Armor +X% and Defensive Value are less important as there are many other sources of improving these values. The exception is Artamark's Star, which gives flat armor values to all damage types. I rarely wear it but I always make sure to have at least one socketed in armor or a shield. At higher levels I tend to prefer Chance to Evade +X% to Defense Value.
Offensive:
Damage +X% (Either all types or specific to the main element the character uses)
Opponent's Chance to Evade -X% (Melee Only) Far superior to just boosting Attack Rating.
Chance Opponent Cannot Evade Attack +X% (Melee Only) Same as above, far superior to just boosting Attack Rating.
Regeneration per Hit (Melee Only) As noted in my pic above, these get their own row in my stash, because they're so valuable. In Ice and Blood, regeneration time for a melee really doesn't matter so long as you have enough regen per hit gear on. With this modifier, it's fairly simple to make it so that melee Combat Arts are instantly ready and recharged, so spam away.
Opponent Level for Death Blow (melee only) Deal much higher damage. Before Ice and Blood, this worked with spells, now it only works with melee hits.
I should probably make more use of the two Life Leech modifiers, since they're additional damage that enemies can't resist but I simply don't use/find them that often.
In keeping with the modifiers I've listed above, these are the uniques I use most often:
Amulets:
Lord Wayne's Wild Rage - provides +all skills and +combat arts
Darwagon's Circlet - damage mitigation and chance to evade
Artamark's Star - flat armor and defense values
Tanit's Collar - damage mitigation and armor lore
Manadth's Fetish - damage over time -x% and opponent's defense value -x%
Gem of Life - max hitpoints and vitality
Vedan's Nemesis - opponent's chance to wound/weaken, etc. -x% and chance to reflect close combat
Rings:
Inquisition Token - opponent's chance to evade -x% and chance for open wounds
Band of Dominion - damage +x% and opponent's physical armor -x%
Ring of Twines - damage over time -x% and block chance: root. great for the swamp
Kybele's Tear - max hitpoints and stamina
Haldorian's Signet - opponent's chance to wound/weaken, etc. -x% and chance to evade
Nyarlothotep's Creeping Madness - opponent level for deathblow and chance that opponents cannot evade attacks
Finally, pretty much all jewelry that's Legendary or part of a Set is bound to be useful. There are 5 Legendary pieces of jewelry (11 with CM Patch), and they're all great. The CM Patch also adds a number of sets with powerful jewelry.
EDIT: This topic reminds of the psychedelic textures for the rings and amulets I found in the game folders. You really only see these textures when the item is dropped on the ground, surprising they went into so much detail for them: