For powerful!Harry, the two fics I enjoyed the most were Resonance and sequels and the Sacrifices arc.

In Resonance, he gradually gets the ability to directly interact with layers of reality, including the dark plane, which in the story is where trully dark magic originates. This leads to fun stuff like summoning demons with a thought (which he unleashes in Malfoy Manor at some point), squelching unforgivables - that is, making them explode on the caster's wand and towards the end of the series, curing all sort of curse damage and even werewolves. Also he can travel to different dimensions.

In the Sacrifices Arc you start with a Harry that due to early years training, he's very competent with magic. In time we learn he is a lord-level wizard (i.e Dumbledore/Voldemort level) and once his magic is unbound he can pull all kinds of crazy shit, not limited to extreme proficiency with the dark arts, linked shield charm walls, creating spells on the fly - but his most devastating ability is being able to drain magic from objects and other wizards and add it to his own. He uses it pretty sparingly, but man is it satisfying when it happens.

Mostly, the reason why I love these two is this: however cool the power-wanking is, it actually serves the plot, instead of the plot serving it. Harry generally gets in more trouble, the stronger he gets. And for all the coolness, magic seldom solves problems, unless it's a problem also created by magic - a general theme in canon which I'm fond of.

 

I loved that one, but I think Hallows and Pathos is better.

If Harry went mad with the stone, why didn't anyone else? Dumbledore didn't.

That's why I like H&P better - anyone who's in contact with a hallow for long enough goes crazy, and the more, the crazier. Except the cloak...which I'm not sure about

 

The Sacrifices arc is something of a special case. It has a lot of things that are common pet peeves in the fandom, to the point that it circles around from bad to being good, mainly because it stops Reading as a Harry Potter fic so that you ignore the HP pet peeves.

I mean, it has wrong boy who lived, abusive parents, Drarry with slash sexual scenes, Snape as a mentor, manipulative!Dumbledore, werewolve communities, Light and Dark sides and magic, magical cores, occumancy via metaphors, a saint!Harry, people listen to him even though he is a child... yet the main criticism I can give it is that it is a HP fanfic. I was a lot happier once I started seeing it as an independent series with similar names for the characters.

That said, it has very interesting/ original magic styles, dark magic that is terrifying and Snape and Draco, even though on Harry's side, are terrible people and portrayed as such which causes conflicto with Harry, unlike most fics in which they are with him. The whole magical core thing is one of the central plot points rather tan just a way to justify them being stronger. There are other examples but Sacrifices is would probably the one that gives the best examples (as in, not as annoying) of everything you hate in the fandom by the simple fact that it is so saturated with them that it no longer looks like the HP fandom but a new story. At least pretty much every carácter in the story is flawed in some way with and they know in-story they are assholes, independently of the side they are in.

Well, at least when it utterly deviates from canon from 4rth year onwards. The whole Harry trying to be a martyr does get somewhat annoying though, even if the whole sacrifice thing (and whether it is necessary or anyone has a right to choose to do so) is the major theme of the series.