Harbinger was pretty dangerous on Insanity. He pretty much instantly possesses any other collector in the field if he's not the only one left, and spams fireballs and his singularity attack over and over to dislodge you from cover. Perhaps he was at his most threatening when he was supporting the final boss too.
My favorite ingame moments with him are when I was losing fights. I know ME2 was touted as having enemies fighting as organized squads against yours but it didn't feel that way too often. But when Harbinger showed up and he was flanked by a dozen Collector drones+assassins, it was pretty stiff. I tended to get a strong feeling of "Oh shit, I'm fucked" when my allies drop and Harbinger taunted "Your allies have fallen, Shepard. We fight as one!" And then, Game Over. I thought he was a pretty fun villain, but a bit redundant and could've used more variety to him.
My other favorite moments were moments of hilarity, particularly when playing as an Engineer. I noticed that AI seemed to react very strangely to those combat drones you can summon, and basically anyone would instantly turn away from you to violently attack that stupid glowing orb annoyingly zapping their behinds. It worked on everything. Harbinger, Krogan Bosses, Scions, Geth Primes, everyone would just completely ignore you and attack the drone when it appears. And since it has an extremely short cooldown, you could basically spam it over and over and lock anything in place and order the rest of your team to tear it apart. Any fight against Harbinger went like this:
"ASSUMING CONTROL"
*lol combat drone*
*Harbinger turns around*
"...I WILL SHOW YOU TRUE POWER!"
*harbinger unleashes powerful explosive attack and destroys drone*
*lol combat drone*
"....MY ATTACKS WILL TEAR YOU APART!"
