Awesome to see other riders on here.
Personally, only you can find the bike that you're comfortable with and feel is the right size. I would suggest something in the 80's to 90's Universal Japanese Motorcycle, maybe in the 400 to 750cc (depends on your size and planned type of riding). There's a lot of great running, reasonable mileage bikes available that are great to learn on.
I started out with an '81 KZ440, moved to a '02 Honda Shadow (couldn't stand the cruiser laid back seating style on the highway for long periods), and just bought a used '00 ZR-7 naked "sportbike". It has a layout similar to the upright seating position of a Kawasaki Concours.