So far I used all of these with included debuggers:
- bochs
- qemu
- AMD SimNow 4.6.2 public
- simics

I used also these but there I did not use debuggers, I used them only to run tests without stepping/tracing the code.
- vmware workstation (win), vmware player (OS X), ESXi server (baremetal)
- proxmox (baremetal)
- virtualbox (win)

I do not develop OSes anymore, but hypervisors, which is somewhat very closely related to OS development.

I'm quite familiar with all of them, with exception of Simics which I started to use only recently. It is not easy for beginners and requires to read the included manual a lot. It is free. It was mentioned at the end this thread in year 2005 when it was not yet free.

Good debuger is a treasure... Not only in os development...
I used olly and x64dbg for windows. I think they wasnt comfortable in xp and earlier non patched by MS win7. From my point of view x64 dbg above 2017 releases are fun, much more usefull. Problem is this 'newest' versions dont work properly on win7 and xp of course - but on win10 I feel difference..

In general: Olly was OK for Pentium4, but not enough for ex. i9. Cpus becomes day by day more complex - programers need more features in tools they use.. And software tools require more modern hardware..