Mythril Sword from Beatrix #1
Fairy Flute from Hilgigars
Demon Mail from Tantarian
Holy Lance from Ark
Grand Armor from Nova Dragon

These items are notoriously hard to steal, due to their 1/256 chance of success. The bolded ones are the only ones I managed to steal, and Grand Armor was probably due to the Master Thief ability.

Which of these items have you managed to steal?