If it's possible to have favorite pyramids, then these are mine. Defining exactly why is a little difficult. This isn't a big tourist site -- the tour buses don't stop here, and you're not going to get mobbed by touts as you get out of the car. This is probably a big part of it. The two pyramids here, both of which were built for Sneferu (2613-2598 B.C.), sit a little back from the edge of the valley, and you really get a feel for the desert. The air is clean, hot, and dusty, and the sky goes west forever.
In terms of historical period, these pyramids come after the relatively crude step, or mastaba, pyramid at Saqqara, and before the fully developed Great Pyramid of Giza, which accounts for the odd aspect of the Bent Pyramid, from which it gets its name: about halfway up, the sides take on a dramatically new angle. The theory is that the builders started at one angle (about 55°), but realized midway that it just wasn't going to work and made a dramatic alteration (to about 44°). When it came to the North Pyramid, the builders used a more conservative 43° angle of attack, and made it all the way to the peak without having to make a change. A steep 30m (98-ft.) climb gets you to the entrance of a low-ceilinged 70m (230-ft.) passage that slopes steeply back down into the North Pyramid (if you've got a bad back or suffer from claustrophobia, head back to the car at this point). The high-ceilinged chambers in the depths of the structure are worth the scramble, however.