I haven't stayed at every hostel in Cairo, but I'd be comfortable recommending Dina's as the best. The room we had, #12, was one of the last ones available when we booked, though looking at it from the outside of the building it looked like the best room in the building. Inside the air conditioning, hardwood floors, and a surprisingly comfortable bed made it even better. This was by the largest room we've stayed in, and the high ceiling with tall doors and windows made it seem all the more grandiose. Dina's has a spacious and bright common area, actually two, for sitting and one with a computer for guest use. The hostel has wifi and free breakfast (with egg and cheese, our best hostel breakfast on this trip), though the best part is the easy going staff that is at the same time laid-back and totally helpful. If you're in Cairo, or ever plan a trip here (which you should, the pyramids are worth it), this is the place to stay. Even on a tight budget you can afford a dorm bed or even a private room, and even if you're used to staying in more expensive (nicer?) places the atmosphere at Dina's alone makes it the clear choice.
Cairo, the first stop in Egypt. Travelers before us had warned abou the dangers, annoyances, and hassles prompted by reduced tourism as a result of Egypt's current political climate, and we were a bit uneasy about the potentially incessant barrage. It was a pleasant surprise when the shops, taxis, camel drivers, and other vendors were pushy but nowhere near as aggressive as we'd been lead to expect. One of the many great people we met was Ryan Harvey, an American musician and founder of the Riot Folk Collective in the US. Unfortunately we had to leave Cairo before his show, but we'll definitely catch him sometime back home. 
Of course the pyramids, just outside Cairo at Giza, were on the itinerary. We didn't make it until before the flight on the last day before departing to South Africa. 
Haggard - Day 65 - Cairo
Looking in the mirror one morning a few days ago Lauren made a comment on her appearance. Looking in the mirror the night we arrived to Cairo I was immediately reminded of the comment, and how her descriptor- "haggard," seemed to fit me a bit better. Of course my first thought was to take a photo out our lastest hostel window. 
Ironically, our attitude anything but matches our appearance. Egypt is phenomenal, and in only our first three days we've met and spent time with so many really incredible people. The food is great, the price is right, and there's not a more historically touristy destination around. We can't help but participate even though we normally avoid a lot of the biggest tourist hot spots. Days and weeks of quick travel, long travel, soap shampoo, and unwashed clothes are no longer a concern. With so much to do here and so many people to see that's not even important, though a nice shower and real shampoo felt pretty good tonight. Now I have to get to bed since it's nearly 4 am and we have an early morning to venture out exploring Alexandria today.