Yes, RICHEST is a Scrabble word!

Letter values in Scrabble US: R 1, I 1, C 3, H 4, E 1, S 1, T 1.

Is it a valid word?

RICHEST is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

possessing material wealth
having an abundant supply of desirable qualities or substances (especially natural resources)
of great worth or quality
marked by great fruitfulness
strong; intense
very productive
high in mineral content; having a high proportion of fuel to air
suggestive of or characterized by great expense
containing plenty of fat, or eggs, or sugar
marked by richness and fullness of flavor
pleasantly full and mellow
affording an abundant supply

Also: full-bodied.