I'd like to see what all others have to say on this subject as well, and would like to share my recommendations or personal experiences as well if they may be of some help.
First off, what is the most important component? Well, that all is going to depend upon what you are implementing your server to do plus the scale at which you are working. For example, if you have a small business with thirty or less end users, setting up a simple server for storage and for domain controller doesn't require a whole lot of processing power or memory to run. However, if that server is also going to be running a pretty large database, plus recording HD video from IP camera systems, then obviously you are in greater need of processor and memory.
Most of the time small businesses are looking for their first server for storage purposes. Obviously then this is an important part and a crucial role of most small business servers. Most of the times it comes down to what level of fault tolerance and performance do you want. In other words, you can get by with onboard software RAID and desktop-class hard drives, but is that going to offer you the level of protection and throughput that you want? I often warn businesses about skimping on the storage side of their server because it is not as easy to go and upgrade in the future. Sure if you need more RAM you can just add more memory, but if you want to later upgrade your RAID controller, you are basically redoing the whole install of your server.
That being said, I don't think that a single one component is ever the most important, but ensuring that you have the right balance of components. This is especially noticed at the storage and network bandwidth limitation. If a small business wants to virtualize several systems on their server and invest in a set of 15k SAS drives in RAID 10, but only have a single gigabit ethernet port for all of their traffic, then there is obviously going to be a bottleneck at the network with how much data can be passed through the network. The processor type or performance, along with memory, are becoming a little less crucial as just about any hardware is capable of running a virtualized environment. It mainly comes down to getting the amount of performance that you need for running your current services and virtual machines, plus room for additional future growth as expected.
Next, what is my preferred Virtual Server OS? I imagine you are referring to the hypervisor for this. For years when you talked about virtualization in the business world the only name that really was mentioned was VMWare. However, with the new Hyper-V included in Windows Server 2012, I don't really see any benefit for VMWare over using the Microsoft product. Hyper-V can be much more cost effective for small businesses to implement: its already included within the licensing cost of Windows Server 2012, depending upon the version that you get. It has become very easy and straight forward to get a virtual machine created and running. One of the things I love most about Microsoft's new Hyper-V is I can create a virtual machine on my Windows Server 2012 system and, in the event of a problem or simply for maintenance or testing, I can copy that VM to my Windows 8 Pro computer running Hyper-V as well, and have it up and running identically as the Server 2012 system. Hyper-V offers great features and performance. Last I had looked the features and performance comparison were actually ahead of what VMWare ESXi offered.
Lastly on this, it can be much easier for a small business with limited or no in-house IT staff to set up and manage their server with Hyper-V than with an ESXi system. You can install the role on the physical server and still access the system directly to manage your virtual machines, plus install any additional standard software you need on there such as backup utilities. With an ESXi setup you need a second computer and the vSphere software or even more complex SQL database configurations to be able to manage your host servers and virtual machines which can become complex for non-technical personnel to troubleshoot or modify.
What applications are we running on virtual servers? It's surprising the things that we can move to a virtual machine which can end up really making a lot of sense and simplify many things. Of course key services or roles operate great in a virtual environment, such as file servers or domain controllers. But many times I have clients who what to also set up a virtual machine with Windows 7/8 for them to install QuickBooks or some other software so they can access a remote desktop remotely without the need for complex and often unnecessary remote desktop services for just a single user sign on. As mentioned before, network video recorders can also be virtualized instead of having to set up completely separate physical hardware for that role. I also personally have tested and occasionally run several turnkey Linux systems that run web-based utilities such as project management or contact management systems. Instead of having to try and worry about compatible hardware and installing on yet another piece of physical hardware, you can set up a very lightweight virtual machine on your existing server to do all the same thing.