NOTE: Windows 7 is not really heavy and has about the same recommended system requirements as Ubuntu does.
1. Sabayon Linux is heavy and has like everything...!
2. Ultimate Edition is heavy (it is Ubuntu with a lot more installed so you might not want to try that).
3. OpenSolaris might be cool
4. Free BSD might be exciting
5. Arch Linux (if you want to "build" your system from the bottom and up)
6. Slackware is mature and stable