Fort William for sure… Just for the variety. You’ve got the Nevis Red, ideal for a competent rider on a hardtail. Also the world cup XC route. And the downhill is hardtailable with a little more skil. (there’s also the waymarked 10UTB route, but it’s a bit pants, it has some good bits but they’re linked by a million miles of fireroad)
I love it there. The DH is a fantastic course on a smaller bike, you really need to work at it and it’s so constant. The Red’s still interesting and challenging, you’d not be that delighted if you’d had to ride to the top of it but with lift assist it’s fab. And the world cup XC has some great features. You can link in Nessie and the first bit of 10UTB singletrack with a little mapreading too.
Not to say that the Glencoe DH isn’t doable on a hardtail, but it’s a tougher course than anything at Fort William, and there’s not much else there. Haven’t ridden the Weasel, but it doesn’t look great.