The best travel lens should be light, cover the long focus range and have good optical quality.

Nikon has a very good lens for the purpose: Nikon 18-200mm f/3.5-5.6 AF-S ED VR II.  The focus length is equivalent to 28-300mm in a 35mm camera which covers the most useful range.  This lens was made of 16 elements in 12 groups.  The minimum focus distance is 20-inch.  Nikon 18-200mm has reasonable building quality and weighs 560g.

The best part of Nikon 18-200mm is the four-stop image stabilization (vibration reduction).  The optical quality is good too.  However, at the wide end (18mm), there are quite pronounced distortion and high vignetting.