Brawl introduced by far the best content in the series imo, at least in terms of new characters, items, music and modes, it was amazing in that regard. I put over 1000 hours combined into it too, it was a blast to play with friends.
I doubt I'll ever go back to it though, because in reality it's extremely clunky and limits the player's freedom. It wasn't something I noticed when first playing it as I wasn't actually very good at Smash when it released, but after getting much better at Melee, and later PM, going back to Brawl it became incredibly clear to me just how bad it felt to play. For all the great content it introduced it failed at the absolutely most important aspect of a game, which is gameplay.