i just bought a 360 last night w/ Halo Reach and Banjo Kazooie.
today, im getting Halo Wars, Gears of War1&2, and Viva Pinata. (theyre really cheap)
but for the expensive games, idk which one to pick:
Fable 3 (what do you do in the game? it looks repeatative. walk for long time, fight, walk, fight)
Left 4 Dead 2 (i dont care for campaign. is there online? what can you do?)
Black Ops (soooooo many glitches. this or MW2?)

which one should i go with?

Elder Scrolls VI Oblivion

You should get Elder Scrolls IV Oblivion. It has around 250 hours of game play just doing all the quest, around 350 hours if you explore the whole land. If your willing to get involved into the game and not treat it like some game to play, and do all the side-quest and such. It's only around 30 dollars on Amazon and look on half.

i would black ops and gta (gta is cheap)

Get halo wars and Gears if war 2. Please dont bother with 1 and for the love of jesus almighty dont get viva pinata.

Black ops is the one that has lots of multiplayer and campaign and especially 3 diffrent zombie maps.

Also I dont recommend fable 3. The campaign is good in the beginning but its awefully USELESS after the first week. Trust me.