What about Duke's? That's a classic.

I second Underhill's reccommendation for Dukes. My husband and I got married there and it is absolutely lovely then. The staff are execellent too and it's small enough that you get very personal service. I think it has been refurbished since then so might not have quite as much of a period feel but is still worth a look. www.dukeshotel.com

Even if you don't stay there, the cosy bar is famed for its Martinis so do try and pay a visit!

Another suggestion is The Milestone. It's part of the Red Carnation Hotel Collection, as is Montague On The Gardens, but at the top of their price line.