I played Ocarina for the first time so long ago that my opinion shouldn't mean anything, but I do think Ocarina was slightly harder than TP. It is like splitting hairs though because they are amongst the easiest games in the series and actually fairly similar in terms of difficulty imo. I think the puzzles in OoT are a little more challenging because dungeons didn't revolve around a single item as much as in TP. The Shadow Temple is a good example of how a dungeon can throw all sorts of random puzzles at you requiring a wide variety of items. I think that kind of game design is more challenging, but once again it's splitting hairs. The only game in the series I would say is easier than those two is Wind Waker.