You are going to get 100 different answers from 100 different people. 😀
For x86 architecture I really like Fedora Core (the free, cutting-edge version of Red Hat Linux). If you're going to run it in VPC, you're going to want the x86 varieties. But it's going to be rather slow running in there.
I would suggest that, instead, you investigate some of the "Live CD" Linuxes that run natively on PPC architecture. This will let you play around with Linux without touching your OS X setup, yet will be much faster than anything running in VPC. I'm pretty sure Gentoo and Ubuntu have Live CDs for PPC, and I think Mandrake may as well. But you'll hear much more on that subject (and many tangents) as people respond to the thread, I'm sure!