1 Zetsu clone is the weakest, however the entire Zetsu clone army at once would be a headache to defeat.
Without counting Zetsu (since he's hard to rank), Hidan is the weakest.
Hidan's main Jutsu is very inefficient since he has to take the time to draw a circle on the ground, collect blood, and must remain inside the circle to use it.
Someone who could crack the ground to destroy the marking would render all the effort useless.
Also his speed, strength, and taijutsu/kenjutsu is Meh at best. Easily outclassed by most Jounin level of Asuma level or higher.
True he's immortal but he could be incapacitated or restrained once he's caught.
*There are also some ways to completely kill hidan:
Incinerate him with strong fire jutsu (like Obito or Enton), liquify him with acid (like mei's), reduce him to dust using Jinton (like Oonoki).