I think all courses have been tinkered with but TOC has probably been “least” touched for the last 120 years and some of it for much longer. So it has to qualify….
https://www.golfcoursearchitecture.net/content/golf-backs-st-andrews-unesco-bidFunnily enough, in Ireland I would nominate The European (it’s Pat’s course and should remain THE example of his work long after he is gone).
I’d include RCD for the beauty of its site and vegetation as much as anything.
Portrush has been significantly changed recently but is a great example of Colt green complexes (although I’d probably pick a Colt heathland or two as HIS examples not to be touched).
Portmarnock has only had window dressing since the 1920’s. I’d be keeping its routing and most of its greens preserved.
Baltray is a great example of Simpson’s work but unsure if it qualifies as HIS best for preservation.
Klondyke and Dell at Lahinch need to be under a preservation order.
Maybe St. Patrick’s as the only genuine Doak example on true links land.
Might pick something from Hackett as well (although I’d probably pick the course where I’ve made a few changes so that is just a little hypocritical!)