Best travel lens for Nikon D3100
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.