Generally speaking, most people in the United States will tell you that good service deserves a 15-20% tip. As a former server myself, I tend to give more than 20% to support my fellow industry friends as a sign of solidarity. Also, I'm proud to say that when I was last waiting tables, I averaged above 25% tips, but that's because I was lucky enough to have wonderful regular customers.
Truly great service can be rare, but in my opinion, there's no ceiling on what percentage of tip your server deserves. I've given out 100% tips on special occasions when the experience was extremely special and personalized.