The last four episodes. Gut-wrenching, screaming, curled-in-the-corner-of-the-couch sobs. Particularly Matsuka's death and the last episode, in which, oh, half the cast dies.
The tearjerkers start way back in about episode nine, with Shiroe's breakdown and death at Keith's hands juxtaposed with Keith's painful confusion about his emotions. Not to mention episode eleven, in which Sam and Jomy finally meet again... only for Sam's brainwashing to kick in, causing him to try to assassinate Jomy, and pretty much destroying Sam's mind. Then the next episode rubs salt in it by showing Keith visiting Sam, who survived, but with his mind regressed to that of a young child. Owww.
Don't forget episode seventeen, which even makes the death of a minor character like Kim into something sobworthy - and that's before we get to Blue's death...
"Tony, are you sleeping? Was it that sad? (...) We'll take you far away, where you can't see Terra."
The ending of the manga is also possibly even more of a Tear Jerker than the anime's, thanks to Grand Mother taking physical control of Keith and forcing him to shoot Jomy, and his meltdown afterwards: "Never warp my will again!" When he discovers the Terra AI that was lying dormant underneath Grand Mother, he barely lets it get through offering him the choice of what to do with it before he begins wearily shutting it down - then calls out to Jomy to kill him.