Mobility - Vayne
Range - Tristana
Damage - Kog'maw
Vayne has short range, but great damage and mobility. [Stealth as well]
Tristana has long range, not so strong damage (as the other 2) and nice mobility.
Kog'maw has medium/longest range [No W/W], the best damage and pretty bad mobility.
So if you want safety, pick Tristana. If you want damage, pick Kog'maw. But if you want a healthy balance of the two, get Vayne.
[In a nutshell anyway]