so i have a 2-year HOTDQ/ROT campaign with a lot of magic items and cool unique skills and healing or bamfing abilities. I am having trouble creating a good challenge for them. I want to really stretch them to their limits and make them use all their resources if possible. They are level 14 currently and are operating on epic scale badassness.

extra info. sorry. Im running in 5e and have 5-7 PCs (depending on scheduling conflicts). 

You could try using a Hobgoblin army, they're pretty week individually but they can be in massive groups so the party would probably have a difficult time handling them, although controlling large amounts of monsters can be difficult.

Another option is to have them fight copies of themselves, you would have to come up a good reason for there to be copies of them, probably some entity conjured them but I can't really think of another reason.

I never consider fights to be all that relevant.  The short answer is, yes, the party will beat the monsters.  I play into that.  When it's a fight, it's time for the combat monsters to break out their amazing abilities and show everyone just how awesome they can be.  Sure, you can have interesting fights, but the stakes need to be something different.  Don't threaten the party, threaten their families, or other innocents.  Sure, the party can mow through skeletons, but can they mow through them fast enough to get to the Orphanarium before the undead can?

One of the best "fights" I ever ran had only one player involved (there were two, but the second was busy).  The monsters Todd was facing down weren't a match for him, especially while he was wielding Warmonger, but, there were a lot of them, and Sherry (the other PC) was busy trying to keep Ana (an NPC) alive as she was critically injured and her vaccsuit was compromised.  Todd knew he could kill the monsters . . . but could he kill them fast enough?  He didn't know.  I didn't know.  He was on the edge of his seat for the entire combat.

Turns out, yes, yes he can.  But, neither of us knew that at the time.

So, you can have an interesting fight, but the real meat and potatoes of the game are the choices the characters have to make.

Will Tabby answer the calling of The Green or will she continue on to complete her father's (Todd's) quest?  Will the Guardian leave his friends when his homeland calls because they will fall without him, and his friends are physically incapable of joining him in his homeland--but by leaving them, they may be unable to save Sherry, and thus, the metaverse may be doomed?  Will the devil give up her aspirations of promotion to do what's right, when confronted with the realization that she now has the choice?

A good character isn't interesting because of their power level, they're interesting in spite of it.

I might be able to help you with regards to puzzles. I am closing in on 50 puzzle videos on my YouTube channel. Each video gives a full demonstration of how the puzzle works and links to handouts if needed.

D&D Puzzles on YouTube - Wally DM

While most of my puzzles are easy to moderate difficulty, there are several ways that you could amp up the challenges if you want. Such as Puzzle #42, Eyes of the Beholder (or #43 Eyes of the Obelisk) they could be in a time crunch to get the puzzle solved while fighting off 3 or 4 Occulo Swarms (Tome of Beasts monster).

Anyways, check it out if you get a chance. Hope you can find something cool and challenging to use!

Two Words: Purple Worm