It depends what you want to shoot, under what conditions (lighting, weather etc) and how you wish to balance ultimate image quality within a narrow range of useage vs lower quality over a wide range of focal lengths.
Do you want to shoot landscapes, wildlife, portraits, indoors, sports, one lens does all etc. etc.. What are your aspirations for the future e.g. will you be needing fast glass, such as f/2.8 and faster? Will your shooting style require IS? What body do you have now? Is it 1.6X crop, 1.3X crop or full frame? If you're on a cropper, do you plan to go full frame any time soon? etc. etc..
Without telling us how/what you shoot the question has almost as many answers as "What car should I buy with £5K to spend?" If we don't know your needs then we can't answer the question meaningfully.