One refurbished Lenovo ThinkCentre M920q Tiny (i7-8700T, 32 GB RAM, 512 GB SSD) running Hyper-V. Small, quiet, and low-power enough to stay on all the time.
Running now: an internal virtual switch LabNetwork (192.168.10.0/24), DC01 on Windows Server 2025 as domain controller, DNS and DHCP for lab.local, and CL01 on Windows 11 Enterprise joined to the domain. A separate Microsoft 365 test tenant covers licensing and user scenarios.
Planned: a second domain controller on Windows Server 2022 (to practise replication and running two versions side by side), a Windows 11 Pro client to compare editions, an Ubuntu machine for mixed environments, and better hardware once this box runs out of room.
The reasoning behind each choice is in Building my homelab.