I'm going to vote other. As others have said there should be a mix of easy-very hard puzzles.
Anything that has to do with the main quest should progress from fairly easy at the beginning of the quest, to roughly medium-hard by the end of the main story.
Side quests, which are optional(duh), should range from medium all the way to extremely hard. I think the best rewards in the game should not only be in the 1-2 hour dungeons, but also have a few medium-extremely hard puzzles. Obviously with a mix inberween for different dungeons, the 15 min ones containing zero-one easy-medium puzzle. Maybe a few small dungeons with very hard.
As long as anyone is able to finish the main quest, I think optional quests should have some very challenging, WTF puzzles. Not everyone should be able to figure them out, because let's face it, the world is filled with dim witted idiots. These puzzles can just let them know they are for sure, lol jk dummies!