I must go further east in both Austria & Slovenia. Nockalmstrasse is the furthest east in Austria and the E61 motorway in Slovenia.
I liked the Nockalmstrasse when in Carinthia, staying at a hotel in Malta. I went over it on a wet day and rode in cloud at one point, then appeared above the cloud.
I went back 2 days later when the sun was shining as I had a tourist pass that meant the toll was free.