The Campaign's DM has always been a big fan of randomly rolling treasure, so I think the rarity will be less of a concern than overall powerlevel.
Of course this is the same DM that let my level 5 character find a, randomly rolled, +3 Shield and boots of flying, so anything may be fair game.
My list so far
Rod of The Pact Keeper - +1/2/3 spell attack and spell DC, recover spell slot 1/day
Ring of Spell Storing, thank you, Falling Icicle, for the suggestion, this thing is amazing for Warlocks. Store 5 levels of spells.
Tomb of Leadership and Influence - +2 CHA
Robe of the Archmagi - Extra amusing when you realise it was on my list before User_Identified suggested it. +2 spell attack and spell DC, advantage verses spells
Since those all require attunement, my list of cool, but less desirable ones I would trade up from
Pearl of Power recover a spell slot of up to 3rd level 1/day
Periapt of Wound Closer Stabilize auotmatically, heal twice as much when hit dice to deal
Weapon of Warning As suggested in this thread. Advantage on initiative and 30ft bubble of not being suprised
Belt of Dwarven Kind +2 Con, Dark Vision, Poison resistance, advantage vs poison, speak and read Dwarven, grow a beard over night
I may seriously put Belt of the Dwarven Kind on the top of my list