As thearkbuilder pointed out in his review, 4:50 From Paddington is the best of the first season of the show. There are the fewest liberties taken with the original novel, and the acting is excellent all around. My favorite cast members were Amanda Holden as Lucy and John Hannah as Inspector Campbell. The minor love triangle that occured between them and Bryan Eastley (Michael Landes) was also well done. You weren't sure who Lucy would end up with!
The mystery is also interesting, and the denouement at the end is almost more effective than it is in the book and Joan Hickson version (though that film is still better). Overall, 4:50 From Paddington is a well-written, excellently acted adaptation of Agatha Christie's classic book.