That would be "The Full Circle," which 1999 fans tend to think is stupid, let alone what the rest of the world thinks. Its hard to say what the most disliked episode is, but candidates include "The Beta Cloud" and "All That Glisters."
As always I point to Martin Willey's Catacombs page www.space1999.net/~catacombs as the best resource for the series online. Willey doesn't give the science problems any breaks, and has pages on such important topics as the Essential Services Hatch http://www.space1999.net/~catacombs/main/cguide/umessentialservices.html and the Multifunction Hatch http://www.space1999.net/~catacombs/main/cguide/ummissionhatch.html
Re: 'the end of the broadcast episodes'
As always I point to Martin Willey's Catacombs page www.space1999.net/~catacombs as the best resource for the series online. Willey doesn't give the science problems any breaks, and has pages on such important topics as the Essential Services Hatch
http://www.space1999.net/~catacombs/main/cguide/umessentialservices.html and the Multifunction Hatch http://www.space1999.net/~catacombs/main/cguide/ummissionhatch.html