I am unsure of your budget, but the nicest letters I have seen on any house, were on the houses at the University of Nebraska. Most of the houses have their letters (about a foot in height), which are raised off the house about an inch or two. The letters are metal and permanently attached to the house. But, behind the letters are neon lighting, so at night, the letters have a color outline and during the day, they are just normal letters.
I wish I had a picture to show you...
Here is one of the ATO house, not illuminated. However, if you imagine it being dark, and the lights would be hidden directly behind the ATO letters, so they light up at night...it really is awesome (all the houses have a color associated with them. I.E., the Beta color behind their letters are red):
