The Collector Ship, hands down, is the hardest friggin' mission in the entire series. At least the first encounter with the ambush on the floating platforms. GAH! After that, everything seemed easy by comparison.
Basically the reason this part is so brutal is your choice of cover is limited and only waist-high, so the two scions which are way off to the sides can rain down fire that forces you to move out of cover or get hit, and all the while the hordes of collector soldiers are more than happy to shoot at you the moment you poke your head up. Given the number of them, there isn't more than a second or two when Harbinger isn't controlling one of them, and he has a lot of instant (or close to it) kill attacks, plus that orb that ignores cover. Also, the platform you're stuck on is stupidly small and your AI squad mates have a hard time reacting to the threats. All in all, it took me probably close to 30 tries on Insanity to get past this part. After that I think the most I died at any given part was 5, in any of the games.
Then again, the next hardest part was in Mass Effect 3 when you're fighting in that London suburb that's being assaulted by endless hordes of reaper troops while the destroyer is getting closer and sweeping the area with shoop-dah-whoop laser fire. It forces you into close quarters with multiple banshees and brutes and on Insanity, even the cannibals burn through your shield/barriers pretty fast if you're out of cover, as well as marauders. I got through this part with more luck and fast reflexes instead of tactics or skill, which always annoys me.
The targeting laser bit on Rannoch against the destroyer is actually really easy when you get the pattern down. Basically as soon as you dodge each of the beams, just stand there and hold the laser on the eye steady, it doesn't move too much that it's a problem and the targeting field is actually really generous so it doesn't have to be pinpoint to narrow. Just stop when you see the reaper's 'eye' glowing and dodge out of the way for as many times as it takes and then just continue holding steady. Even with the last time where it's standing right over you and time is slowing down, don't try anything fancy or prepare to dodge, just hold the laser right on target and don't move.