Sasuke is probably the solo king since he's like the only character who's actually fought a strong opponent 1v1 and won besides Hashirama. Most fights surprisingly aren't 1v1 or are against fodder ninja.
Sasuke beat Naruto 1v1 even though they were both fodder compared to now. And he beat Danzo by himself even after he had been fighting with Gaara and his squad and the Raikage and his squad and the Mizukage.
If not him then Minato since Minato was said to solo multiple ninja in an instant. If that's not solo king I don't know what is.
Itachi has never really fought anyone who 1v1 who was strong and his best victories (Nagato/Kabuto) he had help. So he's really nowhere close to that title if you're being honest about it