Buy the best you can affaord - it will return its value to you with the pictures you take with it
I wanted something a bit special so treated my camera to a Canon EF 100-400 F4.5-5.6L IS USM Telephoto Lens - absolutely amazing lens, hefty price in the UK mind, got it for less than half the UK price by ordering from the states - saved over £700 that way

- Buy it right first time