Illustrious, Ragnos's reign of the Sith Empire is impressive. You say so yourself, correct? And you just stated that Sidious's reign over the galaxy isn't any MORE impressive. So that means you ought to respect Sidious as much as you do Ragnos, for conquering the galaxy. And yet you do not.
The reign in and of itself is more impressive, but that doesn't make him more powerful, or greater, or the best -- you have to look at the circumstances.
As for not respecting Sidious as much as Ragnos, that's clearly because Ragnos is far more powerful. The information we have on Ragnos makes it clear he is the Dark Lord of the Sith, the very pinnacle of the Ancient Sith Empire filled with powerful dark side force users, including individuals like Naga Sadow, who could blow up stars; Simus, who could keep his head alive in a jar; and Ludo Kressh, who was a powerful Sith in his own right. Sidious 'ruled' over one Sith Apprentice, an Apprentice that likely wouldn't even be up to "Sith Lord" rank in the old Empire, and he didn't have to contend with the Jedi, as they had been stupidly blinded by their own arrogance -- the novelization even has Yoda admitting this.
Face it, Sidious took advantage of the circumstances, NO ONE in the Jedi Order had seen a Sith up until Maul, they had no reason to suspect Palpatine was a Sith Lord. For individuals like Kun, Revan, Ragnos, Sadow, the Republic had EVERY REASON to be concerned.
And - Sidious dared to dream of conquering it all. And he did. But no rule is absolute. Every empire had pockets of rebellion in it. Revan's empire did - (with Malak). Malak rebelled against Revan and tried to kill him. No rule nor any reign is total.
In this regard, Ragnos' Empire was more total in its reign. He ruled with an iron fist.
And Palpatine is very arrogant. I know that. It's one of his chief weaknesses. But do not dismiss him because he is a coward. He brought an end to the Republic that apparently - indirectly or otherwise - triumphed over the Sith Lords of old.
It wasn't the SAME republic. That Old Republic had thousands upon thousands of Jedi WARRIORS. When Sadow came around, they launched a massive armada to counter him, and Sadow was defeated only by betrayal. With Kun, they sent thousands of Jedi to beat him. With the Battle of Ruusan, they again sent thousands of Jedi.
How many Jedi did they send to confront Palpatine? 4, and the only decent one was Mace Windu. Awesome "army" there, isn't it?