Glad to see a decent number of people are showing interest in this thread.
I'm the same way, I have a 4x4 and 5x5 but tend to forget the moves to solve them.
Do you have the Rubik's brand 5x5? I have their 4x4 and it's a pain to turn.
From what I understand, Rubik's brand 4x4 is the worst. I got one and it broke almost as soon as I started playing with it.
I'm able to solve up to the 5x5 cube. But best at solving the original.
The 4x4 and 5x5 I'd probably have to re-look up moves to finish them. Plus it's super hard to move the blocks on the 5x5, and it takes me about 40 mins to solve so it's not worth it anymore. It just sits solved on my desk along with the 4x4. But I can do the original in about 1-2 mins.
Never tried any of the pyramid ones or anything other than the cubes.
The Pyraminx is pretty easy. I learned to do it consistently without looking up any tutorials or anything. It's a fun puzzle.
As for the 5x5x5, I'm pretty sure the V-cube 5x5x5 is easy to move (for a 5x5x5). But I haven't personally owned one, so i don't know for sure.
I haven't yet learned to solve a 4x4x4 (purely out of lack of interest, although I'm interested now.)
I also used to be interested in the 15-Puzzle, which is really easy to master. I still have mine, but I don't really have any interest in it anymore, since it's not like the Rubik's cube and other cubes, where you can learn many different ways to solve it.
And then there's the Hanayama puzzles. Which I was interetsed in for a while, but I also kind of lost interest in those.