Zombies are the perfect mindless monster, a great threat for any setting. They're some of my favourite enemies for so many reasons, and that's probably part of the reason they tend to pop up in the games I play. I don't consciously favour them, because they can become tedious unless the game is, for instance, Zombicide, but they're great monsters to fall back on. Because they're such a safe bet, it occurs to me that having a bunch of zombie miniatures for your RPG and wargame is convenient. But it can also be a little confounding once you start shopping for them, because when you play a little bit of fantasy, a little bit of modern horror, and little bit of sci fi, you either have to buy two or three sets of zombies, or else decide what single style of zombie to buy that might fit into all three settings. I have a proposal for a few possible solutions.
Military zombies for modern and sci fi
Modern military zombies and sci fi zombies are often interchangeable, barring outlandishly futuristic space suits or uniforms. It obviously depends on your sci fi setting, but 21st century military attire in a 23rd or 25th or even 41st century setting usually looks fine.
Modern civilian zombies, however, usually look like people out of the 20th and 21st centuries. It's the neck ties and collars that give it away. That works in some sci fi settings, but it can look dated in others. That's the big weakness to using the Zombicide game as a source of a zombie horde. It's full of obviously 21st century civilians in track suits and business suits.
Using modern-day military zombies for modern and sci fi settings is a good solution, but that horde doesn't often work with your fantasy game.
Fantasy rags for everyone
What's truly universal throughout the human experience, fantasy, modern, and sci fi, are rags. Zombies are notoriously untidy. They don't darn their socks or sew on buttons. Their clothes deteriorate, but their bodies keep going. So they end up in rags.
A good set of zombies wearing the shabbiest possible clothes are the perfect zombies for literally any setting. Were the rags once the noble livery of King Arthur's court? Or were they once the high thread-count of a Hallenstein's business suit? Or were they a futuristic space suit? Nobody will ever know, because by the time you get around to asking, the zombie has devoured your brain.
This, for me, is the perfect solution, although it's not without complication. Good zombies-in-rags miniatures can be difficult to find. It's not that they're uncommon, it's just that half of them are hard to paint because they're poorly sculpted, and another third of them are a little too fancy to be generic.
For example, this duo of teenage (apparently) zombies from Reaper is quite good, but at $14 a whole horde would be expensive to collect.
On the hand, these cheap zombies from Reaper are only $18 for 5 but the [relative] low quality of the sculpt could make them hard to paint well.
The amazing sculpts of some Citadel zombies are reasonably priced at roughly $5 for each zombie (depending on your global location). However, they're highly stylized and arguably look a little more fantasy than modern or sci fi.
I don't have a URL for an absolute solution for this. I think the solution is to be ever vigilant. If you see a zombie that looks generically bedraggled, reasonably priced, and sculpted to a quality suitable for easy painting, then buy 5 of them. Repeat the process until you have 4 sets of 5 zombie types, paint them each with a different colour scheme to suggest they're actually unique, and you've got your all-purpose zombie horde that'll last you forever. Maybe even beyond the grave.