Do I even have to say anything? I probably don't, I probably didn't for most of these, but I will. If you skimmed through all of these, you probably saw this coming when I mentioned how I love the series when talking about Playstation All-Stars and it hadn't come up yet. Some people do prefer Melee and I understand that but Brawl in my opinion is just so much better than Melee and Melee is a fantastic game on it's own right. But ignoring that, the reason this game is the best is because it has the best of everything. Subspace Emissary is awesome. The levels are some fun platformer, the bosses are really fun especially the last one, the cast of characters are used really well which is very impressive, and it is just a good story with a lot of badass moments and even some laughs and poignancy.
And honestly, if it just had Subspace Emissary and their exhibition mode, it still probably would've topped the list. It definitely would be in the top five. Because it's like my number two pick, the gameplay is fun enough to carry the lack of modes and as I just detailed, I loved Subspace Emissary. But what makes this number one, is they beyond and above with the modes and variety. There is a lot to do in this game, you've got Classic mode, All-Stars mode, Tournament mode, a boss battle rush mode, Events mode (which is basically objectives based battles), Break the targets!, & Home-run mini-game. I don't even think that's all of them, but if it is that's still a hell of a lot to keep you occupied.
Finally, you can just see how much effort they put into this and how much respect they've paid to the history of Nintendo (and Sonic/Metal Gear). All the music from past franchises, all the characters they've picked, all the levels, all trophies, all the stickers. For instance, here's some Sonic trivia for you guys, the majority Sonic's moveset in Brawl comes from Sonic The Fighters, that terrible arcade Sonic fighting game. And it's such a cool thing. Now, if I had any complaints about the game, it'd be that I wish Mewtwo and Roy made the final product, but all things considered those are nitpicks.
I think I've made it abundantly clear, why Super Smash Bros. Brawl is my favorite game of the new millenium, without any shadow of a doubt.