There are 2 sort of general "types" of Ben Harper songs/styles. There's the folksy, mellow, stripped down Ben Harper with great lyrics and feel, and then there's the amped up, rocking Ben Harper with full band, etc. For the first type, I think Welcome to the Cruel World is the best place to start. His finest songwriting, IMO and a really nice thoughtful album. For the second, Fight for Your Mind, or the newer Both Sides of the Gun are good.
Live from Mars is great, and the interesting thing is that it has 2 discs, split in style just as I mentioned earlier. So, I actually think it would be a great introduction.
PS. The studio recordings are really nicely recorded/produced albums, I really enjoy them on the stereo.