Ooh! Good idea.
MORDIN SOLUS
Wonderful. Love the way he speaks - like electricity! As if he's about to burst - mind bristling, full of ideas. Superbly written, compelling, morally fascinating. Never known a character quite like him. Sherlock Holmes probably comes the closest, but they're miles apart.
TALI... SOMETHING... VAS THINGYBOB
She didn't make a huge impression on me in the first game - where, truthfully, I didn't actually use her (!!) - but I think she's probably the best developed returning character. There are characters like Liara and Wrex who change quite a lot between games - which is fine! Two years is a long time, and it would have been very unambitious not to change them. The story's the richer for it - I'm probably not the same person I was two years ago, so why should these characters be any more static? But while Tali has changed a lot, gained confidence, grown up in Shepard's absence, she's the same in many respects, too. Which makes her feel like she's alive. I think that's bloody brilliant.
MIRANDA LAWSON
What I find really interesting about Miranda is that, though she doesn't change much over the course of ME2, my perception of her totally did. I didn't like her at all when we first met - found her haughty, supercilious, generally untrustworthy, and point blank wanted her off my ship - but by the end, while her personality hadn't changed a jot, I wouldn't really go on a mission without her.
After this point, it tends to get a bit blurred - not a great deal of point assigning specific positions, there's so little between them - but Garrus and Wrex are both certainly in there; albeit Wrex more as the pragmatic loose cannon of ME1 than the level-headed diplomat of ME2. I bloody love Legion, too, and felt horrendous when I accidentally killed him. Joker's a laugh, I thought Saren was very keenly developed... and I'd probably give an honourable mention to that Volus with the biotic powers. Who I encouraged to take on an Asari Matriarch, because frankly, it was a bit of a laugh.
Oh, and my Shepard. Who is female, 'cos I started to make a male one then get annoyed when I found out I couldn't customise my character enough to make him look a bit like myself. My brother did a male one, later, and I think I made the right choice. Jennifer Hale's voice acting is sublime.