Thus far, the Thomas Kinkade 1000 piece puzzle (Buffalo brand) that I completed was by far the hardest. I just started a Charles Wysocki puzzle by the same manufacturer. This brand, thus far from my experience, has the trickiest-shaped pieces.
My last puzzle was a MB brand 1000 piece puzzle. The coating on the pieces made it far more superior, but in terns of a challenge, this puzzle wasn’t as challenging (and all the pieces were more or less rectangular).
I haven’t tried any puzzles under or over 1000 pieces. As far as I’ve seen, some puzzles with over 1000 pieces are too large for me to place on a piece of furniture (sideboard).
Also… I did by the Stow and Go to put away the puzzle when I need a puzzle break, but have not used it yet (it’s mainly a piece of felt, which makes it difficult (read “impossible”) to slide pieces around).