The term 'most guileful' can be defined as having the most cunning or craftiness. The two crossword solutions for this term are 'sliest' and 'slyest'. Both words are adjectives that describe someone who is very cunning and crafty, but slyest is the more extreme version of the two. Sliest implies a certain level of subtlety and slyness, while slyest implies a higher level of cunning and craftiness.