Well, before getting stripped of his implants he was, indeed, a top tier shinobi.
He got defeated by the Uchiha bros, right, but certain circunstances were recquired to beat the guy.
First of all, Itachi would never be able to break free from Edo Tensei by himself. If it wasn't for that misfired KA he would never be available to fight Kabuto. He just happened to have the same trigger as Sasuke (Itachi's eyes, or in his case, an edo replica of them) and had the luck to fight the same guy he stored the crow within.
Second, the aforementioned accident left Itachi with an undying body and unlimited chakra reserves. He would never be able to survive the wounds he got in that fight if he wasn't a zombie, he wouldn't be able to spam MS techniques like he did either because of his otherwise low stamina. In other words, Kishimoto basically removed Itachi's main weakpoint for that fight.
Third, Kabuto's other zombies were busy fighting in a war. You can't just remove pieces from a chessboard once the battle is in motion, it cripples your defense lines and gives an easy victory to the enemies somewhere. So, if he is attacked in a war scenario his best move is unavailable. I will not say Kabuto can control over 30 edo zombies at the same time, since he needs to constantly drain chakra from an external source (like Anko's cursed seal) to do so. However, being able to summon 5 or 6 edo zombies would make a huge difference in a fight against Itachi and Sasuke.
Kabuto is no longer powerful and is back to Jounin level, but you can't just deny he was once one of the most powerful shinobi in the world. He definitely was superior to Itachi and Sasuke, but got defeated due certain circunstances.