Apart from the 1-2 times a match where I get "bee'd" in a bad situation, I don't find the Collectors too difficult. They have big, squishy heads .. the only real threat is playing them on PS3 .. where you are forced to missile in certain waves.
They can be intimidating when you start playing against them, but once you learn their attack patterns and tendencies, they can be nullified quite easily .. a lot of quality head-knowledge has already been shared in this thread, so I won't bother repeating.
However, the more I play this game, the more I find Cerberus to potentially be the toughest faction on Gold. Everything is fine .. until it's not. You can be severely punished for the smallest mistakes against them.
In Platinum, IMO the Geth are the toughest faction (wave 2 ..
) .. if you play them on Condor or London, GL - Stunderdome