Well for me, for anime, since everything I watch is already pretty known, I can't really say that any of them are hidden gems, but I'm currently watching Texhnolyze and it's really neat. It's one of the few series that I find the story absolutely engaging but lacks a lot of other things in animation and execution just because it's a little aged. I haven't watched anything that's hooked me so much on the story in the longest time so that's really nice, and it's not the biggest thing out there yet.

For manga, recently, Bartender has been my hidden gem, but some series that I believe really deserve more recognition as manga that are AWESOME are Gisele Alain and Kono Bijutsubu ni wa Mondai ga Aru!