Worst world for the Space Marines to kill all the "xenos" on?

[Imaginym]

(Perhaps this should be on Fun & Games. I don't know. I'm not sure, really.)

In this case, by "world", I mean planet, generally. But anyway, there are a lot of worlds in fiction. Many of which are home to what the Adeptus Astartes would call "filthy xeno scum" and the like.

So let's suppose, they find such a world, & no matter how strong our Wiki may say those xenos are, the Space Marines end up "cleansing" the world they find, be that seemingly impossible, or laughably easy for them, according to our wiki.

My question is this:

What such world would cause the Space Marines the most suffering, detriment, & overall misfortune, AS A RESULT of them successfully "cleansing" it of all the "filthy xeno scum"?

This thread isn't made for discussing how much trouble the Space Marines' opposition gives them before being exterminated; I'm only concerned with the troubles that would be given to the Space Marines when they succeeded.

 

[Matthew_Schroeder]

What.

 

[Imaginym]

@Matt: What's to be so at a loss for words about? There is such a thing as a pyrrhic victory, no?

Hence, my question, is, what world or planet brings about the worst consequences to the Space Marines if they succeed in killing all alien lifeforms on it, as I've heard they tend to do?

I apologize if I'm a bit ill-informed about 40K.