The Zygon story is heavy-handed and preachy, and suffers from an overly simplistic view towards very complex matters in our own world's current events. But nothing is worse than Forest of the Night. Seriously, that's special type of bad. Worse than Twin Dilemma, worse than Class.
They're both execrable and never should have seen the light of day, Forest because of its reprehensible "hey, kids, it's a great idea to stop taking your meds" message, and Zygon Inversion because of its rather blatant xenophobia (especially with the Zygons being a thinly veiled Muslim metaphor).