Yes, THICKEST is a Scrabble word!

Letter values in Scrabble US: T 1, H 4, I 1, C 3, K 5, E 1, S 1, T 1.

Is it a valid word?

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

not thin; of a specific thickness or of relatively great extent from one surface to the opposite usually in the smallest of the three solid dimensions
having component parts closely crowded together
relatively dense in consistency
spoken as if with a thick tongue
having a short and solid form or stature
hard to pass through because of dense growth
(of darkness) very intense
(used informally) associated on close terms
(used informally) stupid
abounding; having a lot of

Also: buddy-buddy, thick-skulled, wooden-headed.

Not playable in Scrabble US: BLOCKHEADED, LOGGERHEADED.