Shalok said:
Cylons aren't [Cyborgs], they just fake it really well.
^This, which also rules out Skynet as a whole, although the later films do involve at least one true cyborg. I'm not an expert on the canon though.
Also, I think that "biggest threat" is an interesting statement. It could mean in absolute terms - so a "bigger threat" would be the one which could kill more humans - or relative terms - so a "bigger threat" would be more likely to wipe out humanity, or stunt our growth indefinitely.
Finally, people are suggesting the Cybermen as a contender her but I think that's a little pointless given how much more of a threat the Daleks (Who perhaps should have been considered) were shown to be at Canary Wharf