puzzle and dragons is not that hard..you can use gems and place it in any locations on the board..the hardest puzzle game for me is cargo bridge..have to build up bridges to transfer cargo from one place to another without making the bridge fall down..should calculate all the braces and length of the bridges
Colorbind is probably the only puzzle game I literally could not finish because of difficulty. But I'm still working on it, occasionally ill open her up and give her another go.
Lots of levels. I think around 100. Gets SO hard. Try colorbind lite to get the idea of gameplay. (It's probably the most simple but innovative game I've ever played, it's truly brilliant.
Color alchemy - lots of levels, gets quite difficult as well.. Similar to the game flow, you have to match the color blocks to the same color block with lines, but lines can't overlap each other, but it adds many more objects like one object changes the color to the opposite color on the color wheel, another changes it 4 colours forward, another mixes two colours together, etc.. With all these elements it gets very hard.
Woozle- this was a port of a classic console game.. Back in the good old days when console games had so much content. This game has a plethora of levels, and they get very hard even from the beginning. The levels take a while to complete too. At least a few minutes. I'd assume it must take at least 20 hours to completely finish the game.
Hairy balls.. Starts easy for the first 10 levels or so, then gets craaazy hard. Free game too.. But you pay for hints. (Since you want a challenge, you probably won't need hints anyway)
Wriggle - again, it does start easy, to teach you the ropes.. It has around 120 levels, the first 30 are easy to medium, but after 40 the game gets crazy difficult! Free version available - awesome concept, I love it.
Grooh, again starts relatively easy, gets very challenging. Over 100 levels, unique gameplay, head scratching fun. Free lite available.
Penguin patrol, another freebie.. Such a fun game. I enjoyed every second of it. It's medium difficulty to simply pass it, but getting 3 stars is very hard. The game consists of getting to the exit, but each tile collapses after you stand on it. Getting to the exit alone gets you 1 star, saving all the penguins gets you 2 stars, but if you get all the penguins and clear all the tiles and make it to the exit, you get 3 stars.. It'd s huge challenge! Over 100 levels and FREE. I fell in love with this game!
Alchemedes- I recently got this little gem, it's actually been on the AppStore for years, basically your a scientist, you have 3 beakers and you have to get the right amount of liquid into the test tube. You have to measure it out correctly, like you have the bottle with unlimited chemical to start with, then you have 3 empty beakers, each a different ML.. Just say 5, 1 and 9 for example. The test tube needs to be 3ml, so you fill up the 9 beaker, then tip it into the 5 beaker, so now you have 4ml left in the 9 beaker, then you fill the 1 beaker, and you have 3ml remaining. Also, you have a piece of gold floating in one of the beakers too, so you have to work out how to finish with the right mg and have the metal in your beaker you pour into the test tube too. Gets really challenging.. I'm half way through and spent hours on it. There's 150 levels and I struggled after 50.
Zen puzzle garden- this was a classic flash game, pikpok gave it a makeover and brought it to iOS. Again unique gameplay. You are a Buddhist and must prove yourself to the monk by raking the zen gardens correctly. You sweep across the garden but can't over lap your previous sweepings, you have to sweep every space on the grid without getting stuck between rocks or previous sweepings.grest level design, gets very difficult.
Karoshi- instead of most games needing you to survive, it's the opposite, you have to find ways to kill your character. Still haven't finished. Gets hard.
Alio the wood cutter - currently free for the day, definitely get this one, lots of levels and gets REALLY difficult.
Puzzle of the dead - kinda like the adventures of lolo classic nes game, kinda like sokoban but more objects to use, like water, for example. Pushing stones into water to make bridges.
Conways inferno - FREE at the moment, not sure how long for, absolutely brilliant classic flash game, ported to iOS. Awesome classic pixel art, you have to start fires and burn everything down basically, or kill them through virus through water. Really interesting gameplay. I think that's where toast the chicken got their inspiration from.
Ice cream mania- if you ever played factory balls, the gameplay is similar, or gelato mania, or dimo eggs (the last two are free, give them a go, though they are on the easy-medium side) basically you get a canvas, an ice lock canvas, and you get shown an ice cream, for example, a blue ice cream with yellow spots, green lines, and half orange coating. You have to use your stencils to create the same ice cream. Gets very challenging.
Toast the chicken- you must light a fire to kill all the chickens, but you can't get burnt yourself, you have to set the room up with the correct objects to do so, I got stuck on a level and couldn't proceed, it gets challenging halfway through.
Joining hands.. Brilliantly innovative gameplay, you must simply make all the guys happy.. You have a grid and you play "peablins" on the grid but they all must be joining hands to be happy. Each monster has different rules, emos have to be at least 2 spaces away from every body to be happy, some monsters will only hold hands with their own kind, others have 5 hands, etc very challenging down the track. One of my all time favourites.
Save the puppies - don't be fooled by its cute moniker, it's truly a unique and challenging game.
Tractor trails gets pretty hard towards the end, and again, very unique concept
Helsings fire is apparently very hard and good, lots of levels.
Twingo- another one of my favourites. Fun and unique, but not super hard. Medium at best, maybe hard when going for 3 stars on some of the levels. Not a lot of levels either. There are 3 chapters, of 20 levels each I think. So 60 total. Would have liked to see 200 levels total, and have the difficulty continuing to increase after 60 would have been nice. Still one of the best games ever. Reminds me of a game I'd play on Nintendo.
Bag it, same as above, medium, hard sometimes with 3 starring, but absolutely brilliant gameplay. Puzzle mode is really awesome.