What is the best plugins for rendering in Maya ?

mental ray and arnold are great for maya

vray is the one of the best rendering engine for Maya

mental ray / v-ray / maxwel render /

Mental ray

I think mental ray and vray both are good renderers for maya, as well if you want to go more in depth stuff try renderman and arnold for maya too...
It all depends on how you uses the rendering capabilities of a renderer.