Number-wise: The Old Republic. They had the Ancient Sith, the True Sith, Kun's Sith, Revan's Sith, the New Sith, the Brotherhood of Darkness. Ugh.
Power-wise: Clone War+. There's Palpatine, obviously. Then you've got Caedus, Vader, and Dooku. Bane tops the OR era, I feel, what with the KotOR fellas being Unknowns. But Revan's up there too.
Yeah it depends what do you mean by "stronger"?
Numbers? old republic era
Power and combat prowess? galactic civil war and caedus era
Skill? Clone wars and Galactic civil war.
As far as sheer power and skill go i would say the modern sith are stronger, you have got bane, sidious, vader, dooku and caedus.