The Boss Dr. Beat DB 90–expensive but worth it. It has far more features than I will ever need, but I love its ability to subdivide, the choice of click sounds, including a digital voice that will count for you, and the great tempo range.
I have an iphone and for .99 cents the “Tempo” metronome app is amazing. I have it with me wherever I go and I use it every day. Here’s a link with description and review: