if my height is 5'9" what mtb should I buy?
This is impossible to determine via the internet - the fit of a bike depends on inseam, arm length, and a multitude of other physical measurements as well as personal preference and the type of bike (there are large variations in types of mountain bikes even) and riding style. 

Read this article and this article for more details on what goes into bike fitting. Note that its really more of an art than a science.

Go to your local bike shop and ask for a bike fit and try out lots of bikes in realistic riding conditions and find one that works for you.