Anthony Andrews as the Scarlet Pimpernel and also as Sebastian in Brideshead Revisited.
Matthew McFadyen looked more like my idea of Mr Darcy from the books but Colin Firth was perfect personality wise and with Jennifer Ehle they were magic. I agree with a previous poster, that is the definitive adaptation of that novel. At least for my generation.
While I'd never thought of him as blond, I thought Rupert Penry-Jones was a very good Captain Wentworth and Sally Hawkins a lovely Anne in Persuasion; I liked the Amanda Root and Ciaran Hinds version but thought he was unfortunately just a little too old for the role.
Edward Petherbridge and Harriet Walter as Lord Peter Wimsey and Harriet Vane.
Most of the Game of Thrones cast.
Most of the Harry Potter cast.
Sam Heughan as Jamie. I was never quite so sure about Catriona Balfe as Claire, not that she's not a good actress and she's got better as the seasons have gone on, it's just the character of Claire herself that sometimes annoys me. Is there anything that woman cannot do?! Though she is more annoying in the books than on screen.
Richard Armitage in North and South, despite the fact the scriptwriters changed the character arc at the beginning.
Patrick Swayze and James Read in the other North and South (actually, most of the cast in that series).
While the film series were not good adaptations, Shailene Woodley in Divergent. I would have said Theo James as well if they hadn't aged the book character so much that the "instructor romance" thing really made me uneasy.
Another one for Jennifer Lawrence and pretty much all the cast of the Hunger Games.
More recently, definitely Michael Sheen and David Tennant in Good Omens. I wish there'd been more of them, they were so funny and charming together. And I liked the little bit of Josie Lawrence as Agnes. I'd love to see an adaptation of the Tiffany Aching books, but only if they don't turn any of the characters into Americans....although I will make an exception for the Archangel (insert rude word here) Gabriel...