which road is best to take? the upper, middle or lower for scenery, places to stop etc.
You ask train or bus, but if you pick train you don't get a choice, you'll go the coast route.
Bus #100 goes along the coast from Monaco to Nice. Bus #112 goes along the Moyenne (middle) Corniche from Monaco to Eze-le-Village and to Nice. If you want a scenic ride, take #112. Eze-le-Village is one of the most popular tourist spots on the Cote d'Azur.