The mtb is by far my favorite, but I have a road bike and a hybrid. I love rolling twisty singletrack, as long as it's not so steep that I have to get off and push. Where I live it's very hilly so the mtb is best for everyday 10-20 mile rides.
I have got to where long, flat rides don't interest me as much unless there is a destination. Now I look forward to on-road climbs. I like variety in my rides; some pavement, some gravel, some dirt path, hills, and some flat straightaways.
If I could only have one bike it would be an mtb.