Mid.... That's too low. He's at least high but it's debatable that he could be low s rank/low Kage because his data book stats compare to the akatsuki's which consist of mostly mid Kage+ ninja. He just lacks an op jutsu which is y I put him as low kage. Remember, ninja like kisame, itachi, the sannin, Madara, obito, are even high for s rank ninja. Darui and kitsuchi, generals of the divisions had to be low Kage level bc they were the strongest out of all the villages and zabuza fits up there with them.
Also at the Kage summit with the kages body guards, zabuza is stronger than every body guard, but being tied with Darui. The body guards were Fu, Torune, Kankuri, Temari, Akatsuchi, Kitsuchi's daughter, Choijoro, Ao, C and Darui. Zabuza is stronger than all of these ninja, except for being tied with Darui and these guards were the elite of the villages so Zabuza has to be low Kage bc he beats most high jounin ninjas