Virtualization Server Blocks

Imagine you can start over again, the racks of your datacenter are empty and you get back the money you have invested in your infrastructure so far. When virtualizing your datacenter which kind of server would be the most important "building block" ? Assume you are going to virtualize every application in your organisation. Your favorite physical servers would be:
Large Rack servers (Quad, Octal socket)
Dual socket blade servers ("normal" blade servers, many blades per chassis)
Large blade servers (Quad socket blades, fewer blades per chassis)
Dual socket rackservers
Multi-node Rackservers such as Supermicro's Twin² and HP DL1000
Simple Tower servers
Something else (explain!)