The Soviet SS-18 is indeed the most powerful liquid fuel ICBM so far, and in terms of total payload, it is the most powerful ICBM by far, techniquely it can deliever as far as 30+ warheads due to its huge payload.
China's DF-5 is a smaller and lighter-weighted ICBM comparing to SS-18, but dont assume DF-5C has anything to do with earlier DF-5x, it is a completely new ICBM, not a merely modification.It seems to me China will name any liquid fuel ICBM as DF-5 series these days just for strategic disinformation purpose, but the hull, the fuel and all others changed.
Personly I think developing liquid fuel ICBM is still good, since it offer much higher payload, and if China give up the non-first use policy in the future, then large liquid fuel ICBM will give you a huge first strike advantage and can accommodate warheads with very large yielding rate.
Large liquid fuel ICBMs in silo->For first strike
Small mobilied solid fuel ICBMs->For counter-strike.
China need both of them to get a proper nuclear force.