I know this is bit old school, but we have a few older Dell R420 servers that we run ESXi on and use for our own private virtualization testing platform. I've just been tasked with looking into upgrading those servers, but I really haven't paid any attention to server hardware in over a decade (since most of our stuff is completely virtualized). Any recommendations on rack mounted servers to go with (or stay away from)?

I still typically go Dell

This is one of my daily tasks, and I choose Dell over HP as the support for Dell, even out of contract, is available by third parties. Pay attention to Threading when picking processors and the number of processors. You effectively lose half a thread per processor added in the SMP configuration, regardless of the OS used. The Boot only hard disk, get it; it's well worth it, but make sure you have it with backup; no matter which os you choose, get the raid 1 option for this. 10GBe cards, always use Intel, and avoid Broadcom at all costs! Redundant power supplies are highly suggested. Beyond that, the disk is always faster on the system than through any raid or disk array, though I have a bunch of rack configs that are just tiny single CPUs and 2 nic card boxes with shared storage. That and no matter what you go with make sure you get iDRAC

What processor would you go with? While we run a few dozen VMs, none are very high access. It's all internal usage, but they are business critical.

Internal to a team or company of X ppl with regular avg access to the box?

All of our intranet tools reside within various VMs, so used by everyone for various tasks, but we're talking only a few dozen ppl access different things at one time. We're not talking thousands of users. However, we use some of the VMs for data analysis over a large datasets, so performance does matter a little.

I have always been a XEON person since they came out ages ago, for that scenario probably any with a higher cache and 16-24 core

I always go with their upper mid grade offering at the time of purchase, which usually translates to something with at least 20+ cores. Very little difference in terms of AMD over Intel for most applications, other than some AMD boxes do draw less power vs their intel counterparts running identical configs and loads.

Catching up after being away for a few weeks... @dswitzer how many host servers are you replacing?