Overall, the ancient Sith were more powerful.
First of all, the ancient Sith were overall more knowledgeable about the Force and the Dark Side, compared to the modern Sith. There is a reason why the modern Sith keep going to places like Korriban to seek the wisdom of the old masters. Even Darth Sidious himself, who was considered the most powerful Sith Lord in all of history, still had to consult the ancient Sith from time to time. It is also mentioned that Darth Gravid, a crazy Sith Lord who wanted to practice the Light Side, destroyed a lot of valuable Sith knowledge and set the Sith Order back by centuries. Therefore, the Sith that came after Gravid were much weaker.
Secondly, the ancient Sith were obviously stronger than most modern Sith. Exar Kun was capable of draining the life force of thousands of Massassi warriors, and used their power to defend himself against a combined attack from the entire Jedi Order. Tulak Hord single-handedly slaughtered thousands of Jedi using nothing but his lightsaber alone. Naga Sadow used the Force to manipulate a star and managed to destabilize it enough to cause a supernova. Darth Nihilus utilized Force Drain and siphoned entire planets of life. Lord Vitiate also used complicated Sith rituals to drain entire planets to fuel his own strength and immortality.
Honestly, compared to the ancient Sith, the modern Sith were quite pathetic. Anything that modern Sith Lords like Count Dooku, Darth Maul, Darth Vader or Darth Plagueis can do, the ancient Sith Lords can do a thousand times better. The only modern Sith Lord who was comparable to the ancient Sith was Darth Sidious; he did a lot of crazy stuff like the ancient Sith, and he was proclaimed as the strongest Sith in history multiple times.