Proxmox is the obvious answer with your requirements. It really is the "best" hypervisor at small scale, coming from someone who has used both hyper v and ESXI in SMB environments.

The problem with proxmox is that nobody uses proxmox on a business scale, so the experience is only tangentally useful. It's a shame because I straight out think it beats esxi on multiple levels.

If you're aiming to work in a datacentre, oVirt is what you want to learn. oVirt is designed for datacentres and basically is designed from ground up to scale effectively. It assumes from the beginning that you have a clustered storage system and multiple nodes. It's not something you would jump into though until you are very familiar with virtualization tools. It's actually pretty intuitive if you are though.

So, TL;DR; proxmox. It's not gonna be what you use in a business, but it's close enough to ESXI that the knowledge should pass over. It's definitely worth learning hyper v on the side though.