WHM is better than RDM for climbing ranked
RDM is very good, but WHM is better. I also think SAM is better than RDM for climbing since SAM can get those multikill LBs while RDM is typically only going to kill one enemy at a time, even if you land your LB on all 5 on a point. This means yes RDM has better single target kill potential but those won't win matches nearly as often as SAM LBs will.
NIN is also a very good contender because you can solo backlines in their spawn while being ignored. NIN also shuts down RDM and WHM, and they can do nothing back when soloed in the chaotic nature of solo queue.