question

FlexSim will run on a server too but you need to think about licensing, features and the method of access.

"Best possible" will be hard to define since your intended use will play a part as might the diminishing returns on increasing a budget.

Pre-binned CPUs from Intel offer 5-5.2Ghz speed on all cores, while AMD offer more cores often for the same or less money and often will outperform the Intels in massively parallel tests. To my knowledge FlexSim still is not using AVX512 so you don't need to look at the extreme processors that support that.

If its for running experiments you should consider the trade off between the number of cores and the clock frequency. You can roughly say that for experiment with multiple replications, 12 cores at 2GHZ might be the same performance as 6 cores at 4GHz for a given generation of processor. However the experiment on 12 cores will require twice the memory, so when you consider scaling to, say, 50 cores with a model that requires 4GB memory per replication you can see you need 200GB RAM just for the experiment. If you're creating, editing or verifying the model then a fast CPU with 16GB will work well but consider 32GB or more to give you the flexibility to run experiments, depending on the size of your models.

I have found that low latency memory at 3200Mhz will perform as well as 4600Mhz memory with a slightly higher latency even when the CPU clock speed is 5.2Ghz. If you are considering trying faster (than JEDEC) RAM then it's a good idea to check the motherboard qualified vendor/hardware list for compatible modules.

If you intend to load heavy geometry then consider a graphics card with 6GB+ graphics memory (most of the newer ones have this if you can get one), and if you'll also use CAD/solid modelling/video editing tools then consider a professional card like a Quadro class.

Modern hard drives are now PCIe 4 nVME M.2 SSDs rather than the 2.5" SATA variety and will reduce load time considerably.

You should also check the windows requirements and our system requirements.