When the Save America series first displayed for my Senior Thesis Exhibit, I used a solid white Ribba frame that I ordered from IKEA. I ditched the mat, and let the prints fill up the entire frame. As a college student on a budget, they were economical, but they also looked contemporary without having too much flare to distract from the works themselves. These frames would be my first recommendation. A white frame truly allows the vibrant colors of the prints themselves to command the spotlight.
However, a lot depends on what works in your home or office, and every space is different. A space with dark trim and accents may not be suited for a white frame. Consider the space you plan to display your prints. Is it bright and airy? Warm and traditional? Consider the other colors and textures that appear in the room you plan to display the prints. It's certainly easy to get overwhelmed with the variety of frame options out there. I like to think of frames as being the bridge between your space and the artwork. The prints will stand alone, but the frame you choose will help it become part of your home, like it was made for your space specifically.