I vote CBR250R. Lowest entry price, dead simple cheap thumper maintenance with 16000 mile valve checks. 70ish MPG, and very high fun factor, and so light you practically feel like tossing it on your back on the way up to the office. Just a very easy bike to get along with and almost scooterish in it's efficiency and running costs.
If it's just going to be a throw away commuter bike you really can't go wrong with it.