In a doubles or triples combination, which unit is most likely to turn over during a sudden steering maneuver?

49 CFR 383.115(c); AAMVA CDL Manual Section 7.1.2 "Beware of the Crack-the-whip Effect," p. 7-1

This is the single most-tested fact about multi-trailer handling: no matter how the rig is loaded, the back trailer takes the worst of any sudden steering input. It's the reason smooth, gentle steering matters more with every trailer you add.