This app was mentioned in 7 comments, with an average of 2.29 upvotes
CrossMe and CrossMe Colour. Worth the money, I have had the game as long as I have had an android device and still nowhere close to finishing it. These games are better for tablets.
My preferred picross game is CrossMe (there's an ad-supported version as well).
I've been playing fairly consistently since 2012, and every time I start approaching the end of the puzzles, they update with more content.
CrossMe and CrossMe Color. I have lost hours playing these two games. Not sure if it is on Apple or not.
Paid then i'd say hands down CrossMe or CrossMe Color
They've been my favourite with a good balance of difficulty, aesthetics, UI that most of the others seem to lack.
For free then try Hungry Cat Picross, there was something about the rules that irked me a little but it was only minor IIRC and is probably the best free one about.
The others are often held back by a rough UI or puzzle sets, there were a couple (i forget which) that had a decent setup but got into paid puzzle packs pretty quick and by the time you bought them you might as well just waited for one of the CrossMe's to show up on Amazons free app of the day.
crossme and crossme color