Sideous was the most "intelligent" of the sith lords. He successfully manipulated the senete and managed to become the chancellor.
He played both sides skillfully for years (Republic & Trade Fed) until he formed the Empire.
He had nerve, foresight, look at order 66. He deceived the Jedi Coucil from right under their noses!
He used Anakin/Vader's prowess and skill as a tool to destroy the Jedi. Sideous ruled by proxy. He took the Galaxy!
Vader:
You could argue that Anakin was the 'chosen' one, a victim of circumtance and at the will of the force. However, he was still the most flawed sith lord. Not because of his actions as a sith lord, but his actions as a Jedi padawan/knight.
From being accepted into the order at an older age, he was at a disadvantage. Having attachments, fear of losing loved ones etc..
Disregarding the jedi code and wedding padme was dumb and opened him up all sorts of bad things.... just ask Yoda He will tell you.
His failure to see through palpatines lies was dumb.
His decision to attack Windu was dumb.
His decision to pull that stupid flip at Obi-wan and get himself sliced was dumb.
His dection to solo Dooku was dumb (EP II), he got toasted first time, then got his stronger arm sliced off second time.
I could go on... but in essence he was a PUPPET sith lord. Yes, Vader was powerful, but he had Sideous's hand up his back side all of the time.