Climbers going to altitude usually choose a single line between 8 and 8.5 mm in diameter. When climbing on technical and vertical terrain, these ropes are designed to be used as a pair, alternately clipping pieces of protection into one or the other rope in a half-rope configuration, or clipping both ropes into each piece of gear in a twin-rope configuration. On glacier travel, a single one of these ropes can be used to save weight. The Petzl Dragonfly 8.2, weighing just 41.8 grams per meter, is an excellent choice. To have extra length for easier pitches, or to have enough rope to use it doubled up on more technical terrain, choose the 70-meter version.
All-purpose ropes should balance weight with durability. A dry treatment, which helps with water repellency, is also a good option. These ropes used to be available in 10 to 11 mm diameters, but manufacturers have increasingly gone thinner, down to 9.4. The Mammut Infinity 9.5 and Petzl Fuse 9.4 are both good choices for lighter-weight all-purpose ropes. For a little more money, you can get the Infinity in a Duodess pattern, which uses different patterns on each half-length of the rope so climbers can quickly find the midpoint when setting rappels.
Those seeking a little more durability in their ropes, such as guides and wall climbers, will prefer a slightly thicker line rated to hold more falls, such as the Petzl Zephyr 10.3, Edelweiss Axis 10.3, Mammut Flash 10.5 and Blue Water Enduro 11.0. These ropes are available in a variety of lengths, but 60 meters is probably the best choice, as many newer routes have pitches that will stretch a 60-meter rope to its end to reach the belay point.