Presuming you mean most dangerous in a fight without intel:
1. Itachi
2. Shino
3. Suigetsu
4. Hidan
Can't think of any others that really stand out.
Itachi is an obvious #1, Genjutsu off the bat will defeat anyone, regardless of intel, or not.
Shino is next because in the Zaku fight, the insects crept on him silently, feeling his airholes without him even noticing their presence. The Aburame Clan as a whole are really covert operating ninjas.
Suigetsu is after Shino, just because if you don't know about his genetic composition, and you attempt Taijutsu, or close range tactics you're going to die. Asphyxiation kills anyone, that or Suigetsu will submerge himself into their body in his liquid form, then materialize to his full physical form, and cause his victim's body to explode. Nasty.
Hidan is #4, because if you don't have any intel on the guy, and you get cut? It's over. Nothing left to be said.
5-10, I don't know.