What kind of bikes did you grow up riding? If you're only going to have one bike and you place the joy of riding of practicality, perhaps an inexpensive adult version of what you grew up on would be a good choice. For example, I grew up riding BMX bikes (well, BMX bike shaped objects, to be precise). Right now my only bike is a fully rigid singlespeed 29er. It's adequate, though not necessarily great, for a lot of different uses; I do a pretty good job of making it work. But, more importantly, it is the most fun to ride bike that I've ever pedaled. No other bike, no matter how nice or expensive, has reproduced the simple joy of riding I experienced as a kid as well as my 29er. I'm reasonably sure that its similarity to a BMX bike has a lot to do with that.

I'm not saying a singlespeed 29er is right for you, only that you should consider a bike similar to what you had back when you had to have one bike to do it all and enjoyed it.