There's a useful page about 12 key Scottish plays, in the National Library of Scotland Digital Gallery. Whilst you won't find the play texts themselves, there's lots of useful commentary, and other informative audiovisual resources. Clicking on any play title takes you to lots more material about that play.
The Digital Gallery is essentially a portal to lots of electronic resources, based on material held by the NLS. The 12 key Scottish plays is just one of the many resources offered. Maybe you'll think of other plays that would come into your own "top 12", of course. See what you think!