Actual drop rates are unknown, but below is my guess. (Based on my own experiences in playing the game, and what I think is common/uncommon.) Although I have yet to obtain some of the listed items.
UT Drops
1/3 – Poison Fang Dagger, Candy Ring
1/5 – St. Abraham’s Wand, Snake Skin Shield, Fairy Ring, Ring of Divine Faith
1/10 – Experimental Ring, Bone Dagger, Woodland Robe, Captain’s Ring, Coral Ring
1/20 – Cloak of the Planewalker, Staff of Extreme Prejudice, Spectral Cloth Armor
1/50 – Coral Bow, Crystal Wand, Crystal Sword, Conducting Wand, Robe of the Mad Scientist, Candy-Coated Armor, Beehemoth Armor
1/80 – Doom Bow, Demon Blade, Coral Venom Trap, Scepter of Fulmination, Ancient Stone Sword, Pirate King’s Cutlass, Spirit Dagger, Ring of the Pyramid, Ring of the Nile, Ring of the Sphinx, Tlatoani’s Shroud
1/100 – Staff of Esben, Wand of the Bulwark, Corruption Cutter, Leaf Bow, Wakizashi of Eastern Winds, Thousand Shot, Tezcacoatl’s Tai, Wand of the Fallen, Doku no Ken, Murky Toxin, Void Blade, Tome of Holy Protection, Harlequin Armor, Prism of Dancing Swords
1/120 – Water Dragon Silk Robe, Fire Dragon Battle Armor, Leaf Hide Armor, Celestial Blade, Fungal Breastplate, Bloodshed Ring, Sword of the Colossus, Marble Seal, Breatsplate of New Life, Magical Lodestone, Staff of Unholy Sacrifice, Skull of Corrupted Souls, Ritual Robe, Recurring Terror Spell, Tome of Pain, Scepter of Devastation, Hivemaster’s Helm, Plague Poison, Resurrected Warrior’s Armor, Bracer of the Guardian, Skull of Endless Torment, Wakizashi of Crossing Fires
1/150 – Ray Katana, Twilight Gemstone
1/200 – The Forgotten Crown, Omnipotence Ring, Crystal Fang Venom, Seal of Blasphemous Prayer
1/300 – Cloak of Bloody Surprises, Quiver of Thunder, Tablet of the Forgotten King
1/400 – Shield of Ogmur, Helm of the Juggernaut, Orb of Conflict
ST Drops
1/80 – Edictum Praetoris, Memento Mori, Toga Picta, Interregnum
1/100 – Ancient Spell: Pierce,
1/150 – Carved Golem Remains, Etherite Dagger, The Phylactery, Soul of the Bearer, Soulless Robe
1/200 – Pixie-Enchanted Sword, Fairy Plate
I believe that a lot of the ST drops are rarer than standard white bag items because, since they drop from normal enemies that appear multiple times per dungeon, the chances of obtaining them increases greatly. To balance this out, a lower drop rate is needed. This is especially true for Candyland STs because bosses spawn infinitely.
NOTE: A lot of stuff are missing. Again, these are all based on my own experiences and thoughts, so a good portion may be wrong.