Well, no one else has started this thread so I am going to take the plunge.
I love The Talisman Ring. For me it is the first really funny GH novel; of course there are some great moments in earlier books, but The Talisman Ring is a delight from start to finish. In character terms it has a couple of GH firsts: the first 'decoy' heroine - we expect the wayward Eustacie to be our leading lady, but of course it turns out to be the delightful Sarah - and the first world-weary, apparently unsympathetic but ultimately totally sound hero in Tristram.
The lesser characters are also particularly fine. Who can forget the rakish Ludovic, the fierce old man Sylvester, the cold-ridden claret enthusiast Sir Hugh Thane, the sinister Beau Lavenham or the laconic Bow Street Runner (the first of several in GH novels, all cut out of the same cloth)? And who can fail to laugh at Ludovic's ridiculous appearance in drag or Sarah's doomed attempt to appear knowledgeable about panelled strapwork? Well, not me anyway.