Just as it is in, the magic items in are organized into rarity levels to designate their strength, ranging from uncommon, to rare, to very rare, all the way up to legendary.
Generally, an item's usefulness can be determined by comparing its rarity to another of the same type; for instance, a +2 rare longsword is a straightforward upgrade to a +1 uncommon variant.
But there are quite a few items for which that is not the case – items that work particularly well with a specific build or are otherwise strong enough to overshadow even those of higher rarities.
These uncommon magic items are more powerful than their rarity gives them credit for, and hold up even against higher-level items of the same type.
Found in the githyanki crèche during Act One, the Strange Conduit Ring is a good option for those that combine martial and magical might. It adds 1d4 psychic damage to the weapon attacks the wearer makes, as long as they are maintaining concentration on a spell while they do so.