Vile Swarms are the best monster type in the game due to the following factors:
- they come in large packs
- they are very fast (for kiting)
- they give good progression for their HP
- they do not have a collision box
Especially the last point is very important here: you can stack them up infinitely tightly just by kiting them around in circles, and make it very easy to obliterate them with very few attacks due to insane Area Damage scaling. If you find many of these in the same place, every single Greater Rift map turns out to be great because all you need to do is blast them within a few seconds and move on. It's worth noting that these enemies are also considered flying and move over small obstacles or holes in the ground or even the edge of the map in some cases (e.g. Realmwalker map).
For more details about Area Damage, check out this guide by sVr: Area Damage Mechanics