, I've had a wee look and can see that for St. Andrews to Stirling, the most scenic route would be this one
. Going through Dundee and Perth, alongside the River Tay.
For Stirling to Glencoe, I'd recommend this route
. This looks like the most scenic option, taking you up past a couple of Lochs and through some lovely villages along the way.
For the final leg of the trip from Glencoe to Oban there is only one route (according to Google Maps) but it takes you along the west coast, which will be a great sight. Here's a link to the route
Let me know if this helps, and have a wonderful trip!