Golspie is a definite must, it’s amazing. It’s a simple up and down, but the climb is really good fun and not in the least bit steep until you hit the top section. The down is amazing, 7k from top to bottom.
Laggan too, pushes the boundaries of trail centre grading.
How about the Fort William DH and World Cup Trails? DH is really good fun and long, and the World Cup and 10 under the Ben loops are good fun and worth it especially if you’re in the area.