I'll start with an answer.
I guess I first saw him in "Midnight in Paris" as F. Scott Fitzgerald and I thought, "Well this is quite a 1920ies Dandy!"

Then I saw the Avengers (before I watched Thor) and I was fascinated with his
charismatic Loki. What an entertaining super villain! I thought there would never be a super villain like Heath Ledgers Joker again, but this Loki is indeed somewhere in that league, too, I think. (Though of course he is different in many ways, compared to the Joker.)
And since that Avengers movie, I've watched everything with Tom Hiddleston I
could find and listened to some audioplays, too. I like his voice a lot.
And I enjoy his interviews very much because he has so many interesting and often deep thoughts to tell, and not only about acting or his roles or his colleagues...

So what about you? What was your first impression of Tom?