All are good RMx comes with reasonable price tag and HXi is the Platinum series with huge price tag. All are great if you are having normal budget and not spending crazy amount on the build then I would recommend RMx. If you wanna spend crazy amount then go for HXi as it will give you bit more stability over RMx.
For example HX750i and RM1000x are priced nearly same. In that case I would go for RM1000x as it will be more stable being less stressed than HX750i(having lot extra watts of power is never bad as it gives better stability due to being less stressed).
If you have crazy budget and can afford HX1000i then only I will recommend to go for it.