Anything that makes you want to ride it. I realize time and money are tight but try test-riding a few different kinds of bikes (rigid mtn bike, touring or cyclocross bike, road bike, hybrid) and see if anything calls out to you. You might wind up checking craigslist or consignment shops for something that will fit into your budget and still leave room for peripherals (helmet, rack, repair kit...)

Even if something calls out to you now your opinions might shift over a few k miles of riding... I would guess a rigid mtn bike is most likely to work for you right now and will never be useless in your closet if you do decide you'd more enjoy riding something different. People I know who are not gearheads stick with their mtn bikes forever.