There are tougher enemies, but the husks in ME2 are bloody annoying when the get close- particularly on Insanity when a good short shotgun blast isn't enough to take them down. Like the headcrabs in Half Life, it's not their damage so much as their size and general anoyance factor. The rest- Harbinger, Scions & Praetorians- just needed the right squad mix, a good strategy and patience. ME2 got a whole lot easier when I stopped playing it as 3rd person shooter and deployed my team powers effectively.
Of all game villians, Regenerators from Resident Evil 4 and Feral Ghouls/Reavers from Fallout 3. Consistently skin crawling no matter how many of them I ran into- creepy, tough to kill and horrible, HORRIBLE sound effects. That unmistakable ghoul greeting as I crept through a disused DC museum or factory always gave me goosebumps.
And forget the vision, the Regenerator fx got me every time:
http://www.youtube.com/watch?v=6H7HeJKPCN0&feature=related