YMMV / Darwinia

• Best Level Ever: The Biosphere, which requires the player to command an army of Darwinians to wipe out and seize control of the facility from the Infected Darwinians one incubator at a time. Humongous Darwinian battles abound, and the whole stage feels a lot more RTS-ey than the rest of the game, not to mention that, aside from a few viruses on the central island, your only foes are other Darwinians, giving you more of an even playing field.
• Demonic Spiders:
  • Soul Destroyers which are very hard to hit since they fly around. Also, they destroy Darwinians' souls, which prevents you from bringing them back as new units.
  • Of course, giant spiders. They are so tough that the only way to kill them quickly is by using explosive weapons, but their favorite combat maneuver is jumping into close combat, so you have a hard time killing them without losing some of your own units to friendly fire.
• Game-Breaker: Engineers can move anywhere, and capture Control Towers as long as they remain unmolested long enough. And resetting a level doesn't reset the Towers. This means distracting higher-level monsters, such as Soul Destroyers, with a Squad while your Engie captures a tower is a viable strategy.
  • Progress toward capturing a tower is also not lost, which means if the Engineers get even a tiny bit of work in before buying it, you'll get the tower eventually. And the tower only has to be 50% captured for you to start spawning Engineers or even Squads in atop it.
• Goddamned Bats: Spore Generators due to their ability to float out of danger after getting hit, Triffids because many of them are placed on remote locations and Army Ants for their multiplication properties.
• Good Bad Bugs: When you save a level, the save is not an exact state save, and glitches can occur as a result. On the Yard level specifically, the existence of individual ants is not recorded, only the anthills that spawn them. Thus the requirement to destroy all ants (as opposed to all anthills) on the level can be met as trivially as exiting and re-entering the level.
• Nightmare Fuel: Every single virus. You thought mobile arrows weren't intimidating? Take a good look above and be proved wrong. EVERY SINGLE VIRUS has some hellish shrieking whenever you hurt them. Not only that, spiders are HORRIFYING in how they just jump on you.