"This is a very sad song, but, if you're like me, I always find the saddest music is also the most beautiful. And this is one of my favorite songs that I've ever written, and it's called Stop Swimming." —Warszawa Live Album
"Blackest Eyes" is nothing short of overwhelming... but when you look at the lyrics: Oh Crap.
It's remarkable how such a gorgeous song can have such cutthroat lyrics: 
"Heartattack in a Lay By"? Steven Wilson's mournful voice, the final verse of the song, and the sad keyboard make this the saddest song ever. The guy in the song had a fight with his wife and goes to cool off in a rest stop, but dies of a heart attack before he goes back.
"I guess I should go now She's waiting to make up To tell me she's sorry And how much she missed me I guess I'm just burnt out I really should slow down I'm perfectly fine but I just need to lie down We'll grow old together..."
The beauty of the album that song is from is that all the songs are pretty ambiguous and can be interpreted any way you want, but the song takes on a darker tone when you think about the fact that it's on a concept album about psychopaths and serial killers. Especially when you learn about Fred West, suddenly the last four tracks on the album — including this one — seemed to fit in with the story of how West was caught and brought to justice.
While the music from Heartattack in a Layby is incredibly depressing — the ending verse of "Collapse the Light into Earth", the last song on the album, can be more heartbreaking.
"I won't heal given time I won't try to change your mind I won't feel better in the cold light of day But I wouldn't stop you if you wanted to stay..."
Throughout the whole song, Steven says "Will" in a way that it's interchangeable with "Won't", so it can be taken as optimistically bittersweet or just a straight downer.
"She changes every time you look By summer it was all gone - now she's moved on She called you every other day So savour it it's all gone - now she's moved on"
The video for "Way out of Here", which is dedicated to the founder of the PT myspace group, who was killed by a train.
Most of the songs on Fear of a Blank Planet, but particularly Sentimental. There's something heartbreaking in the way Wilson sings:
"I've wasted my life And I'm hurting inside I don't really know And I'm not really sure
Sullen and bored the kids stay And in this way they wish away each day"
Not to mention "Buying New Soul" or "Where We Would Be". Just saying. Oof.
Especially the solo in "Where We Would Be," so full of hopeless longing that it aches just to hear it.
The very reminder that we'll all die is beautiful, heartbreaking, and horrifying all at the same time.
"Normal", "Stop Swimming", "Shesmovedon", "Black Dahlia", "Feel So Low", "Arriving Somewhere But Not Here", the ending of "Anesthetize", some others depending on context... probably the majority of their music. Then again, Steven Wilson, the main songwriter/leader of the band, feels that the most beautiful songs are the saddest, so this makes sense.