What is the best kind of bike for riding up hills?

The best bike for climbing hills would depend on what the surface is. Basically for climbing you want a bike that has the right gearing range, is as light as possible, and still being able to cope with the surface you're riding on. If you're riding on roads the best climbing bike would be a road bike with a triple crank. If you're riding single tracks the best climber would probably be a cyclocross bike with a triple crank. If you intend to run the bike downhill at some speed and on a rough surface the you'll probably want an XC or AM mtb instead. It'll be a poorer climber but it will deal better with the descents. IF you want to get real technical bikes with short chain stays generally makes better climbers off road. With more weight directly over the rear wheel there's better traction. For riding in loose sand you can't beat a Surly Pugsley with Large Marge rims and Endomorph tires. They're far wider than just about any other tire out there and will let you roll along on the surface where skinnier tires gets bogged down. If that's out of your league money wise, look for just about any MTB with the frame clearance to run wide tires. Go with UST (tubeless) tires and rims which will let you run lower tire pressures than the standard clinchers. Low pressure will give your tires a wider foot print on the sand, with better traction and better ability to stay on top.