Dandy Dungeon: The Legend of Brave Yamada
This is great. It's Yoshiro Kimura's (former Square and Love-De-Lic employee, director of Chulip and Little King's Story) first independent effort, and it's definitely my favorite free phone game. It's a grid-based dungeon crawler with a meta-twist; it has a really solid gameplay loop, which consists of drawing your way through the floor of the dungeon and engaging in auto turn-based battles with the enemies you pass through. There's no ads, and I was able to beat it without spending any money. It has tons of loot and outfits and items and optional dungeons and daily and seasonal events---it's a pretty meaty offering.
Mostly I love it because it's just really strange, crass, and entertaining. It's full of goofy characters and designs, like Yasu, a suit-clad child who looks up to Yamada; an Indian coder who's such a master laptops levitate around him; a giant two-faced asparagus monster that emerges from the floor; and Yamada's love interest: the much younger Maria-chan who loves sweet potatoes. It has a real goofy sense of humor that's totally in line with many weird Japanese games from the 90's and 00's, which is suitable given Kimura was a part of so many.
It's full of surprises and it's free. If you like weird Japanese games (Eno, Suda, SWERY, Nishi, etc.) you owe it to yourself.