My favorite not so realistic gun is the double Desert Eagle from The Green Hornet.
For more normal guns it has to be the Beretta 92 that is in many movies but it really caught my eye in The Boondock Saints. When I saw it in that movie I had to have one. I love it....
Funny thing is that I was just reading on IMFDB that in some scenes the double Desert Eagle is just a Beretta 92 with a slide on each side of the 92's slide.