Count Dooku and Obi-wan vs. Darth Sidious
In one of the best scenes in Episode II, Count Dooku makes an appeal to an imprisoned Obi-wan to join him and destroy Darth Sidious together. In actuality Obi-wan refused, but let’s imagine he did decide to join Dooku.
Obi-wan is missing, presumed dead. For the next 6 months as the clone wars begin Count Dooku trains Obi-wan in secret for their planned confrontation with Sidious. Dooku concentrates on improving Kenobi’s bladework but his appeals for his new apprentice to draw on the dark side go unheeded. Dooku resigns himself to improving Obi-wan’s power with the force within the constraints of the light side, while continuing to utilise the dark side himself (and pointing out, to no avail, how much more powerful he is by using it). The training is specifically geared towards confronting Sidious, so Dooku spends much time showing the best methods for dealing with force lightning and impresses upon Obi-wan that he must be prepared for Sidious’ incredible speed.
The actual confrontation occurs on Coruscant. In a scheduled meeting with Sidious in the works, Dooku and Obi-wan openly approach Sidious together. There is no surprise attack as such, beyond Sidious’ initial surprise that his apprentice is just about to try and kill him. In this regard the initial setup resembles the confrontation between Dooku and Obi-wan/Anakin on the Invisible Hand. Given that the confrontation occurs in the works, there is nobody else around.