Motorcycle Off Road
What is a good On Road/Off Road Motorcycle That's Street Legal??
Completely depends on how much dirt riding you'll be doing, and what type of trails.
If you really want a dirt bike that can take an occasional 10 mile scoot on pavement between riding areas, look at the offerings from KTM - or convert a 250cc to 450cc four stroke dirt bike using a Baja Designs http://www.bajadesigns.com/ kit (not legal in all states).
If you want the bike more for exploration - dirt roads but not much single track. Ocassional tough rough sections or sand washes that require some muscle to get the bike through - this type of riding (think Baja) and about 30% street use, then the Honda XR650L, the Suzuki DRZ400L and the Kawasaki KLR650 (not the '07 model) will fit the bill best.
Now, for 'adventure' type riding - take off for weeks at a time carrying your tent and everything else with you. Roads to dirt to rough unmaintained two-track. This is what the BMW GS models are made for. Ducati and KTM also have a complete lineup of models for this type of riding.
The best bike is the one that suits YOUR style of riding best. Decide what you want the bike to do, then pick the bike that does those things best 