Depends if it's unupped or upped.
Unupped it's Monofire. Upped it's Monodark. Death can compete well with both, both unupped and upped.
These are the unconditional spots they have in the meta. If you start introducing environments, more specialised monos like Aether can dominate completely without any real competition at all, whereas monos like Fire and Darkness are strong rather because of their versatility and won't have a 99% winrate against any mono.
If you're starting out, you'd be able to use Monofire the best, because it farms AI the fastest. Honestly though, you can easily pick Fire and improve it by buying some rainbow stuff, à la decks like Ubah Stompy, for instance.