I’ve been playing Eternal X which I’ve found fun. It’s what X and Y should have been which is to say it’s still fairly vanilla Pokemon with just enough tweaks to really help bring a lesser appreciated generation more up to speed.

Pokemon Unbound is a gen3 romhack with updated mechanics and pokedex up to gen 7ish. When you start, it gives you 3 different options to make the game more challenging (difficult, expert, and insane). It also gives other options, like level caps for each gym, randomized abilities, and a “scalemons” option where every fully evolved pokemon has 600 base stats. I just recently did a run with scalemons and random abilities at the expert difficulty, and it became a really fun challenge of crafting a different team to beat each gym leader. And since I wasn’t doing a nuzlocke, the cost of failure and trying again was very low. Lots of side missions and postgame content too.

This game has somewhat spoiled me, in that I wish every other romhack I play had some of the things Unbound has

It’s not a romhack but I’ve really been enjoying pokerouge, I’ve found it pretty challenging. It’s a browser game. https://pokerogue.net/

radical red is pretty choice, as well as any drayano hack. there are text and guage animation speedup codes available for gen 4 games to help mitigate the slowness.

I’m almost done playing Pokemon Odyssey, it probably isn’t as hard as hacks like radical red or kaizo emerald but it is definitely a lot more challenging than any original game. Unfortunately it’s only ~75% complete right now but it’s definitely one of my favorite hacks already

If you haven’t already played it, definitely play Rocket Edition. It’s not designed for increased difficulty, though I found it was a bit more challenging than vanilla. But the writing is phenomenal.