18-200 is the most awesome lens for travel, because carrying more than one lens and swapping while you are out and about sucks. That being said I did my entire last trip with a 16-50mm, which was good, and after using 16-50mm for the past few months, I don't really feel I am missing much from the 100-200mm range. However having just a bit more range on the 16-50 would be good, so 15-85 would be a very useful length.
TL : DR IMO 15-85 is good length.