If I could have only one bike it would be a mountain bike.
I rarely ride more than 25 miles a day, which is practical with a MTB. I like to ride on trails, alleys and gravel roads as well as being on the streets a lot--usually wihtin the same trip.
The MTB is rugged, better in bad weather IMO, good for heavy loads and more adaptable than some other bikes. You can easily put fenders and panniers on one. (but check the kick clearance for panniers, as some MTBs have a short wheelbase.)
Downside--it's hard to maintain speeds much over 16 mph, so not good on those long road commutes.