The Wraith, Shadow, The Poisoner

[Corporal Rubbadub]

Spoilers for Redwall, Outcast of Redwall, and Salamandastron
In Outcast of Redwall, when Swartt Sixclaw surrounds Salamandastron, he hires The Wraith a weasel who is excellent at small-knife throwing, and very good at stealth, climbing and camouflage. In Redwall, Shadow is a weasel-rat who Cluny has get the tapestry of Martin, also very good at camouflage, climbing and stealth. In Salamandastron, Farran the Poisener is hired by Ferahgo the Assasin to poisen the food and drink of Salamandastron.
I would say Farran is the best out of these 3. he slipped past Long Patrol Hares inches away from them. The Wraith is next on my list. He climbed more than halfway up Salamandastron! He would have assasinated Sunflash if not for a rock otter cream pie that the hare cook threw at him by mistake. Shadow is very skillful, but third on my list. He made some mistakes on his way like shutting the door to loudly while going into Redwall, and checking for intruders while biting off Martin from the tapestry.