I am partial to Californication and Stadium Arcadium because I am partial to the guitar work of John Frusciante (see my av), but Blood Sugar Sex Magic is probably their best work, imo, with Dave Navarro at the ax. A very good guitarist in his own right. Can't go wrong with a Greatest Hits album, though I haven't seen what's actually on the disk.
Personally, I like "By the Way" the best, even if it didn't get any love. But I'd start out with the Greatest Hits, like aforementioned, and see where you want to go from there.