What are your recommended games for Xbox 360?

I bought my first Xbox 360 a couple of days ago and I'm searching for a few awesome titles. I already have a PS3 and Wii, so I'm focused more on first- and second-party games but if you know of any good multiplatform titles that'll be appreciated too. And because I have Gold I'd really like to know of games with a strong, and thriving online community. My budget is kind of tight too so if there are any value games feel free to tell me about them. Thanks!