Perfect Dark Zero... and yes, I did regret it. Luckily I got GRAW soon after and all was well.

Mine was NBA 2K8.

PDZ, amd a soccer game

My first game on the 360 was G.R.A.W.  It was pretty cool.  I remember activating invincibility and infinite ammo and just blowing up cars after I beat the game.

Saints Row. It was one of the reasons i wanted a 360.

Saints row was to hold me off until SCDA worst purchase for me it was extremely repetititve and bland

Oblivion, one of my favorite RPG's :)

Mine was Saint's Row. My girlfriend at the time bought me my 1st 360 with Saint's Row & it was probably the greatest birthday present I've ever received. Well, my 360, I mean. Although, I completely dig Saint's Row & I'm totally excited for the 2nd one.

Gears of war was my first.

Mine was PGR3, I went to a midnight bash just to pick up the game, ready for my delivery the next day.
Then I played Hexic.

Gears of War, I bought the system for the game. It did not disapoint.

DoA 4

For me, it was Oblivion and Perfect Dark.  I still play both, though the sneaking missions on PD tick me off something fierce.  I hate games where I'm forced to sneak around!

Saint's Row it was.

Rainbow 6 Vegas.  My brother already had a 360 and I played the game at his house.

Perfect dark zero and condemned, after waiting for 12 hours at Wal-Mart for the system launch

marvel/ forza

Call of Duty 2. Got my 360 soon after launch. But my favorite first Xbox 360 game I didn't expect was Kameo. That game was really fun, can't wait for Banjo now!

mine came with Kameo and PGR3, but the first game i actually bought was Bioshock

This thread needs to be stickied

I got to play the RRoD game first.  After that got shipped off and repaired, I played Perfect Dark Zero. 

My system came with Marvel Ultimate Alliance & Forza 2. I ended up buying/playing Gears of War first. That definitely set the bar rather high for the 360.

Viva Pinata!

NBA Live 2006.

Halo 3, and I have played it nearly every day too. I love that game.

GRAW1

Gears of War.

My first 360 game was GRAW. aww what a great time I had playing graw online. Made a lot of cool friends on it. but most of them i took off because they stopped coming on or didn't play the games I were playing. 

My 360 came bundled with Forza 2 and Viva Pinata, but the first I bought was Mass Effect.

Rainbow Six Vegas.

Mine was Tony Hawk's Project 8, and soon after I got Call of Duty 2.

Project Gotham 3.  I still love that game.  I got that, Perfect Dark Zero and Kameo on launch.  PGR3 still easily takes the cake as the best launch 360 game.

First game I played was Hexic. I must've blown 2-3 hours on that alone before I bothered opening the games I purchased with the 360.
Those games being Full Auto, Fight Night Round 3 and Oblivion.
First out of those I played was Full Auto.

Perfect Dark Zero.
I liked it, not so much anymore though.

Perfect  Dark Zero. I wasn't happy with that game.

Lost Odyssey, it came in and I am not even buying my 360 until next week >_>

I think it was Ghost Recon: Advanced Warfighter or something..

Saints Row, then Gears of  War.

I got Halo 3, Gears, and Oblivion all at the same time. Before I even had a 360, actually. Toys R Us was having a buy 2, get 1 free sale right before I got my 360, so I went and nabbed those ones. Halo 3 was the first I played.

I got GOW with my 360 on my birthday, then a day later on Christmas got a few more games(R6V, COD2 which was free, and FEAR I believe). Christmas Eve B-day FTW.

i dunno with the 360.....Hmm..

but i do know the old xbox - it was Midtown madness 3, came with the console

Kameo and PGR3

Need For Speed Most Wanted. It was pretty cool. Then I played Perfect Dark Zero. Not as cool.

Oblivion, and it kept me occupied for a month before I felt the urge to buy something else.

My first game was Marvel Ultimate Alliance , because it was with my 360, but I got Halo 3 two days later...

It was Perfect Dark Zero or Condemned. Both decent games in my opinion. Once better came out I haven't gone back to Perfect Dark Zero.

I tried to figure it out my first game based on when I got my first achievements.

Call of Duty 2.

I got Condemned before the launch, but I came home from the launch with the box and Perfect Dark Zero and played that.

Got Perfect Dark, PGR3 and Kameo at launch.