Given that MAME is (unless something has changed recently) still heavily reliant on single-threaded workloads, the Core i9-9900KS would probably be the ultimate CPU for the job (it has the best single core performance of any x86 CPU ever made) Although there are probably very few games in MAME capable of running at playable speed on a Core i9-9900KS (or other similarly highly-clocked CPUs) but that can't run on something more modest like a Ryzen 7 3700X or 3800X.