With out a doubt the best place you can take her is The Rim Rock. Everything else is a distant second. In second place and still respectable would be Hys, Quatro or The Grill Room.
The Rim Rock would be more suited for a romantic Valentines dinner, with still having a busy atmosphere. Food is fantastic and you will gain huge points by knowing to order the Sticky Toffee Pudding for desert.
Your only problem is going to be getting a reservation.
Skip the Bearfoot Bistro, or Araxis.