I'd be much more selective than the OP, but one game was not mentioned so far, and that shows me how 'hidden' this 'true' gem indeed is:
Everblue 2. A wonderful game of the shamefully neglected underwater genre, great game design, great missions, the "pacing" (to use a fashionable word) is perfect, collecting and exploration (two basic important elements of game design) are at times brilliantly executed. Worthwhile having, worthwhile playing, a gem of every collection.