Something in the 10KΩ range usually works pretty well.
You aren't very specific with the needs. How far does it need to turn? 90˚? 180˚? 270˚? 360˚? More?
From my experience so far in the 'real-world,' Bourns potentiometers are the gold-standard.
You may also like to take a look at US Digital Rotary Encoders, specifically the
MA3
We had good luck with the MA3 in the past. It can output as a 10-bit (ignore this... it works great) analog value. In essence, it works the same as a potentiometer, but has no physical rotation limits. The voltage will go from 0V to 5V, then reset to 0V.
Next time, give us some more information. We're smart people here, but we can't read minds.