Broky shisui is dead,minato isdead,any ninja ho got i advantage over kabuto are dead,the holy ninja that can kill kabuto is tobi,,,he got rannegan...How can you say there's a #1 ninja when they don't all have the same powers.
They have their own set of abilities that can prove very effective against certain ninjas and not so effective against certain others.
When it comes to a battle of who dies first, I would say that Kabuto's weakness would be someone who could teleport instantly to him and kill him, seal him, etc.
This itself won't be an easy task considering that he has alot of stolen power up his sleeve. In my opinion, Kabuto's weakness is Tobi since he can warp away Kabuto or his Edos to force Kabuto to a 1v1. I haven't seen Kabuto summon Edos that are already outside of their coffin fighting so I won't include this here.
It'll be a problem though if Kabuto starts the match hiding. Well unless Zetsu's spores play a part here and Tobi is able to sense Kabuto through this. It also hasn't been stated whether Tobi himself could sense Kabuto with Zetsu's spores or not so I won't include this here either.
Overall, to beat Kabuto in a 1v1, you only have to get rid of Kabuto while surviving the onslaught that the Edos will give. What person could do it better than someone who has the ability to transport himself and others away to another place?
Then again, Shisui also has a good chance if the battle starts head-on since it seems that Danzo was able to control people using Shisui's sharingan without even having to show it. Shisui could just control Kabuto without Kabuto noticing or a KA could be an instant defeat. Danzo also has a chance and if Itachi has Shisui's eye in his crow, so does he.
Not many people have an advantage over Kabuto but I still think Tobi has the best chances.
Tobi we no rannegan he is kabuto meal