Tali was amazing, S'roka was phenomenal... whether aiming to be funny, dramatic or shy. A faceless voice to carry all emotions? Seriously, most characters have an emulated face which reacts accordingly, she did not.
Martin Sheen made TIM's role his own. Seriously Kudos to him.
Garrus was as top notch as in ME1, some of my favorite conversations revolve around him.
Legion had a synthesised voice.. which possibly makes him illegible to this but he makes Legion sound really cool, I'd say he's the main reason people like the Geth.
Miranda did a great job, not sure if it's Stahovski's first VO job, but she worked with many pro VO actors and did a really good job.
I think that Fem and Male Shep's VO's get reduced appreciation as they are meant to be less emotionally resonating as their colleagues... I believe that they are intending to give the Player leeway when it comes to emotional engagement and if their voice sounds a way that counters the player's intention, it could hurt the game... hence they always sound... "less intense" - If I may say so.
To be honest, I could go on forever, talking about all of them... but my work day is over and I want to go home
All in all, ME2 is like ME1, top VO acting, emotionally compelling and other adjectives whom could serve to only emphasise the awesomeness of it all.