Just while I'm at it:
That'd be Apple II on Mac OS 6 on Mac OS 7 on Windows NT on Mac OS X... All emulated. Not virtualized.
Total abuse of a PowerPC chip, but honestly they were gluttons for punishment...
[Edit: This picture is from years ago, if that wasn't clear -- it's Virtual PC on an old G4 running OSX 10.3, not Parallels or similar on a more modern machine. With hypervisors today you could nest a whole lot more than this, as long as you stayed virtualized rather than emulated (i.e. all host OS's running on the same architecture). Emulating across architectures tends to reduce speed by a literal order of magnitude -- SheepShaver, for example, runs at around 1/8 speed.]