Which virtual machine is the best?

Depends on the virtual OS hardware requirements for CPU and RAM.

I don't absolutely want to revive a dead thread but this is something that plagues me too often since some solution always has some weird problems so you can't use it... I agree with you if you mean Oracles Virtual Box as it seems to do its job quite well and with good performance. With VMWare I always tend to run into limitations where I would need to buy a better version.

The "best" depends on what you are trying to emulate and what your host platform is.  I agree about Virtual Box and VMWare.  For free Virtual Box works pretty well, unless MacOS is your guest OS then it takes hacking as I understand to make it work.  The frustrations with the free version of VMWare are undoubtedly entirely intentional to drive sales of the fairly expensive commercial versions.  The commercial version is pretty powerful though, but expensive.  If your host is MacOS then Parallels may be a good option, especially if MacOS is also the guest OS.  If your host OS is Linux two options besides Virtual Box and VMWare are KVM and QEMU.  If your host is Windows then Microsoft's Hyper-V is an option albeit non-free and as far as I gather it can't run MacOS as a guest OS.  There are others out there but these are the main options.

 I use Proxmox for everything here, is based on Debian Linux, very versatile and free. Supports ZFS, but is memory hungry. 

https://en.wikipedia.org/wiki/Comparison_of_platform_virtualization_software

I stopped using Oracle Virtual Box but it is easy to use. I have an old copy of Microsoft VirtualPC for PowerPC to run Windows and it was terrible.

I have used QEMU to run operating systems that use PowerPC and x86 CPUs.

I use a lot of emulators and in doing so I use Wine in order to run AppleWin to run Apple II software. I also run KEGS to run Apple II software. Mame is a really amazing emulator that I use for non-Apple II emulation. I find Mame difficult to set up but once it's running it does a superb job. There is no "best" emulator just a lot of emulators that are better in certain respects. I've built my own emulators, simulators, and virtualizers: Apple-1 for Apple II, Isaac (Isaac Simulates An Apple Computer), and currently I am using my own 6502 simulator that I consider a VM or at least that's how I am using it.

Recently, I've been using DOSBox to run Turbo C.

I've built Linux From Scratch (LFS) which uses chroot if that counts.

Oh and there's cosmopolitan libc.

I guess it depends what and how you want to virtualize.