We recommend Wittner and Taktell metronomes. They are precision instruments made in Germany. They have a swinging pendulum that helps with counting.
Piano students should get a Taktell Piccolo or a larger Wittner pyramid type metronome.
Avoid the smaller models as the tempo increments are hard to see and adjust. They are not as loud as larger models and will be overwhelmed by the sound of the piano.