I've used Sixt at the Aberdeen Train Station a few times now - they are always kind, helpful, and accommodating. Naomi and the other staff have been very friendly with any questions or post-hire issues. They are also flexible with late drop-offs, which is helpful if the highland roads add a bit of time on your journey!
My only issue with Sixt is not being able to contact the office directly (as comms or lost-and-found items must all go through their central site), but any time interacting with the Aberdeen staff has been very pleasant.
All in all, would recommend and will continue to hire from them!