I'd agree with Meryl Streep as giving the best performance by an actress this past year that I saw. I suspect Helen Mirren in THE QUEEN would be up there if I had seen it, but that's one that will have to wait for HBO/Showtime/Starz.
I did adore Mirren in ELIZABETH I, but that's TV rather than at the movies.
As for the men, none of the highly touted acting performances from this past year have I seen: DiCaprio in two films, Damon, Forest Whitaker in THE LAST KING OF SCOTLAND, Peter O'Toole in VENUS. So I can't make a choice.
I did like Hugh Jackman in THE PRESTIGE, and Eddie Murphy is sensational in DREAMGIRLS, but it's a supporting performance.