The honest answer is that the best beginner bike is the one you are not frightened of. Everything else - displacement, brand, styling - matters less than whether you will actually ride it.
Ergonomics
Sit on everything before you buy anything. You want both feet reachable at a stop, bars that do not stretch you forward, and a weight you can hold up on a camber. A 350 single at 400 pounds is a different machine to a 650 twin at 500, and the difference shows up the first time you paddle backwards out of a parking space.
Scooter or motorcycle?
If your riding is under ten miles at a time and mostly city streets, a 150 scooter does everything a small motorcycle does with less to learn and nothing to shift. If you want to leave town, you want gears and a bigger wheel. Plenty of people here own one of each.
Which one is right for you?
Come in and tell us your commute, your budget and whether you have ridden before. We will put you on two or three things and let the bikes make the argument. We would rather sell you a 350 you keep than a 650 you list for sale in the spring.