Halo 3

After the Second halo ended with such a cliff hanger and no so much a great ending people were wondering if the 3rd Halo would make the ending work. And guess what? It did. Halo 3 had a great Campaign and gives you many difficulties to choose from, from the kids that aren't that good at FPS' to the Halo Freaks that want the most difficult missions to deal with. 
But in my option what makes this game a 5 is the multiplayer. It is the easiest to just pick a play list and just start. Also it is easier to play with friends then Gears of War and to a lot  easier to play ranked games with friends then GoW. Another reason why this games is so popular is because there are so many play lists to choose from. From the serouis "MLG" to the fun and not so seroius "Rocket Race". Also you can now change your armor to your choosing to look different. You get different choices by getting achivments.
There are some new weapons and equpiment. The new weapons are the Gravity Hammer which is the weapon the last boss in halo 2 had. It is a weapons that just crushes everything in site. Another weapns is the Spiker with is a brute duel wield weapon that is sort of similar to the needle. The needle is now stronger and can not be duel wielded. The mauler which is another brute weapon that is very strong but is a close range weapon. It can be duel wielded which is just as powerful as a shotgun.
The equipment is a new feature in halo 3 but is not use in MLG.
List of Equipment
Power Drain- Drains the shield of the Spartan and stops vehicals in its trackes.
Bubble Shield- A bubble forms around you which not gun or granade can go through but Spartans can go in and out.
Regenerator- When in it Shields go back up.
Flare- Blinds the spartan.
Radar Jammer- Shows a bunch of red dots on you radar so you done know were anyone is.
Spike Granades- Can Stick Spatan and shoots out spikes
Fire Granade- If hits the person becomes a flame and dies.
Others- Gravity  Lift, Trip Mine.
Other things to note. The is not a missle pod that lockes on to vehicals.  Flamethrower which shoots fire and can insinerarte the Spartan. And a turrate can be riped out. All three of these things bring you into third person mode and you walk slower and can not crouch. 

Therter and Forge.  Therter is a new feature that lets you watch the game you just played. When playing you can take screenshots and video clips which you can add to your fileshare whichs lets everybody view. Forge when released was not expected to be something used so much but now is one of the most popular things that people do many maps for MLG and even makeing racemaps.

So Overall Halo has a great Campain but what makes it for me is the multiplayer which gives you amazing options and many things that every person will find at least one thing they like.