First choice would be stainless steel-handled rescue. Strong, has a proven lock that I trust, cuts things that other knives can't, and has plenty of what I buy Spydercos for - SpyderEdge!
Second choice would be stainless steel-handled ProVenator/ProGrip II with 50/50 blade. Versatile utility blade, small enough to be legal in most locales, and comfortable, sturdy handle with a good grip.
You said only one though...