I counted 8 game modes (multiplayer) in regards to Gears 2.  I counted 10 (plus the Forge stuff) in Halo 3, googling.   The word FAR more is pushing it in regards to his.  Forge is great, but Gears 2 has enough in comparison.  And is able to be played single player and offline.  In the case of Halo 3, it is either play online against oher people or trade it in.   I traded in Halo 3 after beating it.  I still have Gears 2, due to the bots.

My take on Halo 3 is that is one of the most hyped FPS's in history.  It is a competent title, and the Halo series deserves praise.  However, it isn't all that and then some.  I believe Gears 2 replaces Halo 3 on the quality front.  Gears 2 is superior to Halo 3 in just about every way.  I also consider Gears 2 should be the XBox's flagship 3D shooter title, replacing Halo.  Halo is still the top NAME, but as far as 3D shooters (FPS and TPS) go, Gears 2 would be the new flagship title.

All this being said, Halo is still king, and remains such, not even having Gears topple it.  This should say miles to anyone trying to hype up KZ2.  If Gears 2 didn't topple Halo, then Killzone 2 won't.  Halo was the first top-flight FPS to appear on a console, and set in place the template for how to do it.  And this makes Halo remain the top.

After reading this thread, I can conclude that "Halo Killer" is a taught thrown out by XBox and Halo fans to Sony fans daring them to say a title on the Playstation would top Halo.  They are just waiting for someone to do it, so they can then pounce whenever the title hits the market and doesn't do better than Halo in sales.  The original Killzone probably is the source of this taunt as there were likely some Sony fanboys who hyped Killzone over Halo and called it a "Halo Killer".  I know one guy who actually prefered Killzone 1 over Halo.  When pressed, it was because Killzone didn't have vehicles you drove in it.

Game modes in halo 3 officialy sponsored by bungie off the top of my head:

Team Slayer, Juggernaut, Brains, Swat, Team Snipers, MLG, Team Doubles, Neutral Bomb, Oddball, Crazy King, Legendary Brawl, Rocket Race, Griffball, Infection, VIP, Squad Battle, Capture The Flag, One Flag, King of the Hill, Swords, Team Hammerzeit, Big Team Battle, Shotty Snipers, Territorries, Team Fiesta, Save One Bullet, Team Shwatguns, Multi Team

anyways like i said that was off the top of my head but that is 28 modes.  If there is one thing Halo 3 is known for its the endless of amount of changes that can be made to rules to create new game varients.  These are all approved by Bungie as official Matchmaking types and aren't modes only played in local multiplayer

A bunch of those have to be new additions to Halo 3.   I know Griffball was not in the game orgnally.