I voted for the paper 2x2's. For metric readers: these are 5x5 (centimeters).
In the US, there is also a 3x3 (7.5x7.5) carton frame. However, as far as I know these will fit only in US (three ring) album pages. Also, 5x5s come in pre-glued and stapled varieties. I use pre-glued, even though they're much more expensive because I hate the look of the staples. I used Pritt sticks for a while, which works fine but is still messy.
Peter