I love dearly "Shop Around the Corner". Sure it's dated, but it's so warm and human, all the cast have their moments to shine.

Another favorites are Desk Set with Hepburn and Tracy, How to Steal a Million and Clueless.

From the 21 centure I have a weird choice for a favorite movie - "What's your number?" with Chris Evans and Anna Faris. The actual plot is horrible, but the leads work so well together that I enjoy watching it anyway.