Kaku was the second strongest member in CP9 with a Doriki level in the 2,000 range, who Zoro fought on par with.
Lucci was the strongest with a Doriki level in the 4,000 range, who Luffy also fought on par with.
Luffy is obviously stronger than Zoro in the CP9 Arc, as well as most other instances, with the exception of a few cases, like debatably during Part II without Gears.