Yes, RANKEST is a Scrabble word!

Letter values in Scrabble US: R 1, A 1, N 1, K 5, E 1, S 1, T 1.

Is it a valid word?

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

very fertile; producing profuse growth
very offensive in smell or taste
conspicuously and outrageously bad or reprehensible
complete and without restriction or qualification; sometimes used informally as intensifiers
growing profusely

Also: out-and-out, right-down.