As it has been told many times in these forums, there's no "best mic". It's a matter of personal taste and of the specific characteristics of the singer's voice. But specificaly about the strength of the output signal produced by the microphone, the stronger the better. With a hotter intrinsic signal you'll need less pre-amplification (the pre-amps produce noise) and you'll achieve a better balance between the keyboard sound and the voice. From this point of view, the SM58 is not the best choice amongst dynamic mics. In the same brand (Shure) you have the Beta 58A with a strong output level (in the specs, look at a parameter measured in mV). This works well for me, but I haven't tried every mic! My best solution (that I'm presently applying in my live acts) is to use an external pre-amp (actually a small Behringer feedback killer with other useful features included - and I can also use condenser mics that require phantom power) before injecting the signal in the keyboard mic input. This way I keep the keyboard mic gain to a minimum, hence reducing the hiss, and still obtain strong vocals. Since phantom power is no problem, I'm using a condenser Beta 87A mic, and I'm getting great results with it.
-- José.