Ask HN: What emulator would you choose for evolutionary computing experiments?

Context: I would like to try "evolving" an operating system from the first byte onward. The architecture doesn't really matter as long as it's suitable for general purpose computing. I'm guessing the most important feature is startup time, as waiting 5 seconds to run 5 milliseconds before crashing would make for slow progress. There are so many emulators out there (and mostly hobbyist quality, it seems), I'm not even sure where to start evaluating the options. Anyone here have experiences or wisdom to share?