Convert AmEx MR points -> HHonors, PC or M&M ??
 

Here's the scenario. I usually convert my MR points to HHonors or Priority Club points at par (1MR=1HH or 1MR=1PC)

I also could convert them at the same ratio to Brussel Airlines (SN) Privilege miles also at par, but so far it has been "who cares?". However next October Privilege will be integrated into Lufthansa's M&M and all Privilege miles will be transferred to M&M. This is much more interesting.

So what's worth more? 1 HH or 1 PC point or 1 M&M mile? (50-100k points in all)

i'm guessing you have MR(UK) because MR(US) doesn't have Brussels airlines listed :(
imo M&M miles are more valuable, it all depends on what your potential flights though