First off, I'll admit my bias. I know the story of Harry Potter pretty well. The only other story found in Lego games that I know is Star Wars. So if I didn't enjoy Pirates of the Caribbean or DC Super Villains as much as this, that might be why.
But that said, this one is by far my favourite of all the Lego games. Why?
Firstly, the way in which abilities work here is different to a lot of the other Lego games, where only one particular (class of) character can destroy silver objects, where you need another to use telekinetic powers, or another to open locks. For this, most characters can do most things (provided that you've done enough of the main quest to get them).
And I like that. Some things are gated (for instance, dark magic, goblin keys, Parseltongue, Weasley boxes, broken object repair), but most aren't. Especially nice given that one of the abilities (strength) is also available from an unlockable red brick. So you can take your favourite character and stick with them.
And then there's the music. That's more due to the source material than the game itself, but I like the soundtrack a lot more than years 1-4. Sorry, John Williams. But I like the later films' music a lot more than what you did for the first four. It seems more fitting.
The characters still talk in "hmm" and "ahh", which I actually prefer to the fully voiced games seen later on.
Finally? Luna. She's not in the first Lego HP game. She is in this one. And she even /skips/ when she walks. I rest my case.
As for the game's performance and stability? I haven't had any issues. I'm running it under Linux (using Wine, it's not native) and it's been flawless. No crashes, no glitches and solid performance. The strange corrupt lighting that I got when using Lumos in LHP 1-4 doesn't happen any more and the graphics are just prettier in general.