Halo 3 is as close to perfection as games get.
Let's begin with campaign. It's got a difficulty for every person. It cannot be too hard, or too easy (if you find legendary easy, you have played too much). Also, the computer players are actually somewhat helpful, at least in easier difficulties. The enemies are not your typical same guy over and over. There are Hunters (Harder), Brutes (Average), Jackals (Easy) and Flood (Ugly). And of course, Grunts. These guys are worth buying the game on their own. They're hilarious. Finally, one word: co-op. Grab a friend (even online) and kick some alien a$$.
There's also matchmaking. What's more fun than Red vs. Blue with 16 people? Nothing. The only problem is those people with competitive egos who ruin it for everyone else. This could be considered that one flaw, but every game has this issue.
Third is multiplayer. THE best part. You can do your regular Slayer matches, but also other games, like Oddball or Infection. You can even make up your own game. Like low gravity Gravity Hammers! And there's awesome weapons for any game. Rocket launchers, energy swords, sniper rifles and Spatan lasers. And then there's the vehicles. Nothing like a three man joy ride through the desert in a Warthog, plowing down anyone you don't like.
Then there's Forge. What else is there to say. You can make your own maps! That's amazing! You can make the ultimate map that you've always wanted!
Finally, there's the video function. You can watch your silly videos over and over again, and save them, to show someone later.
I cannot find a flaw with this game if hard core fun is what you're after. If you're not after fun, then you suck. It's got great single, online and multiplayer, you can make your own maps, and even your own videos. I may have put 9.5 above, but I feel that Halo 3 is about a 9.9.