This answer might change with the winds but in competitive play Togekiss and Jirachi are some of the most annoying bastards in the cosmos. They paralyze you with Thunder Wave and then because of their Serene Grace ability their bread and butter attacks have a 60% chance to Flinch you and they're also tanky and have strong recovery in their movesets and they just sit there and death by a thousands cuts you

For Shiren it's either Weed Men (all levels) or Kappas (level 3+).
Weed Men (also called Field Knave, Field Raider etc) are weak in combat but they're item destroying freaks. The first form just wanders around destroying ground items and turning them into Weeds. You run into them pretty early game (5~8F) when ground loot is the most important for getting your run off the ground. The leveled up forms have greatly enhanced abilities, such as throwing Weeds into your pots, throwing Weeds to knock items out of your inventory or even directly turning your inventory items into Weeds.

Kappas are also item destroying freaks, but instead of fucking with your inventory they throw ground items at Shiren and the high level ones do so from incredible distances and the effects of these items do activate when they hit you. Level 4 Kappas can throw items at you from anywhere on the floor and they don't even need to be able to see you!!!