which microscopy is most useful for living cell?

If you wish to keep the cells alive, then a light
microscope is the right tool to observe with. Light microscopes transmit light through
the materials being viewed, and then through a set of lenses that magnify and focus the
image. Because the light can get hot, care must be taken to keep the cells wet while you
are observing them. A variety of stains are available to accent certain features of the
cell or increase contrast of the view overall; however many of these stains are toxic
and will kill the cells. Because the light is transmitted through the specimen, this
type of microscope works best with materials that are naturally transparent or are very
small or thin.

Electron microscopes get much higher
magnification than light  microscopes do, but they require that the cells undergo a
stabilization process that will kill them; therefore electron microscopes can only give
you a "snapshot" type view of the cells.