Most systems are going to say they are simple for staff to learn, especially the front of house staff.
My company does a few POS systems, I don't think we're in your area though but certainly can connect you to someone.
NCC Reflection is a fairly easy to use system. Feature wise, it is probably middle of the road. With a backoffice or cloud reporting, it makes it more complete.
POSitouch is a full featured system, competes with Micros and Aloha. Interfaces with almost everything out there. You see it in Cheesecake, Uno's, Outbacks, DQ but also in a lot of smaller restaurants and groups.
Dinerware might be a middle of the road one to look at although I think Heartland bought them. I would stay away from systems that lock you in for credit card processing. I would also avoid any POS system that isn't making a profit yet. You never know where these guys will be in a few years.