another options could be bronzong, probobass, dialga or bastiodon.
Bastiodon and Probobass are not very good at attacking, but good be annoying with some threating moves, and they both resist rock-type moves, which are very common in OU.
Bronzong is a another good option because its Leviate ability and restitance to rock moves. It can learn Earthquake, Stealth Rock, Rock Slide, Gyro Ball, Payback, Trick Room, Reflect and Light Screen, which can be useful to protect or cover Fire types. Especially Reflect is useful, because it neutralize the ground and rock weakneses of your Fire Pokemon and in combo with Light Screen or Sunny Day to cover the water weakness.
Dialga is another good option because is resist rock and neutralise water type moves. It learns Earth Power and Acient Power, and it can learn Earthquake, Stealth Rock and Rock Slide, which are pretty coverable with your Fire types.
In OU, Dialga would be the best choice, but otherwise Bronzong will do very well. Bastiodon and Probobass are good ingame, but in OU they do not very well.
If you don't like my prefered choices, than go with Heatran or Empoleon