You should focus on the four general resources—processor, memory, disk, and network (that are the main elements that are virtualized). In order to have good data, you should gather these metrics for a minimum of one working week, and preferably over a one month period of time to match and discover all the different usage and peaks.

Of course, in order to have good and representative data, it's fundamental to gather those metrics during critical business cycles (such as payroll processing, batch operation, reporting, or other business processes) where performance may spike. That data and the peaks will help to right-size your environment and define the right consolidation for your workloads.