It was a tough choice. I loved Matthew Macfadyen too very much and was delighted by the movie in general, but I voted for Colin in the end, because I fell in love with his Mr Darcy first.
At the moment I'm actually focusing on other works of literature more, almost in the same fanatic way as P&P, but P&P will always have a very special place in my heart.
I love Matthew McF and watch any of his movies that I can, but I thik Colin Firth knocked a lot of socks (and perhaps some other items!) off women around the world and I think is unsurpassed in his portrayal of Mr. Darcy.
I've voted in this about 10 times now. I vary some of my other answers, but I always pick Colin as my favorite Darcy. And I'm not above helping to stuff the ballot box if it'll get him a win!
For all those P&P fans out there who prefer Colin's Mr Darcy, here's an interview you might like. Unfortunately, the video has been deleted from the Internet for copyright reasons, but you can hear his lovely voice and read the transcript.