What alien bits should I sell in XCom 2?

You always need 1 corpse for autopsy/breakdown. You also have a minimum number of corpses needed to unlock instant autopsy/breakdown, which means that, if you want to save research time, you will only sell corpses after the autopsy is done. An exhaustive list of other uses for corpses is:

• ADVENT Trooper: 2 for Hybrid Materials (4 on Legend), 6 for Predator Armor (12 on Legend), 2 per EXO Suit (4 on Legend), 2 per Nanoscale Vest (3 on Legend).
• ADVENT Stun Lancer: 2 per Spider Suit (3 on Legend).
• Sectoid: 2 for Advanced Psi Amp (3 on Legend), 1 per Mind Shield (2 on Legend).
• Faceless: 2 per Mimic Beacon.
• Viper: 2 for Battlefield Medicine (3 on Legend), 1 per Nanomedkit (2 on Legend).
• Berserker: 1 per Overdrive Serum (2 on Legend).
• Chryssalid: 2 per Hellweave.
• Gatekeeper: 1 for Alien Psi Amp (2 on Legend).