Here's a few suggestions from different stages:
If you like to see how great artists progress in their career, then why not start at the (almost) beginning, with The Freewheelin? Simple, protest folk style with powerful songs. You may find this one endearingly authentic, or embarrassingly dated.
If you're only going for just one or two albums, then skip the transitional albums and go for his confident, all electric, rocker Highway 61 Revisited. Arguably his greatest. Aggressive, punchy style.
Or for a softer, more ballad style, then Blonde on Blonde, with long, long, songs with outrageously oblique lyrics that go on for seemingly thousands of verses. Paints great images, even if you have no idea what he's singing about. Not for the impatient though. This was originally a double LP, which now fits onto a single CD.
Of his more modern output, then try Love and Theft. A no nonsense return to basics. Ditto Modern Times.
A couple of wild cards (of many great options): Nashville Skyline for an early melodic, country style. Not his greatest, but it's very easy to listen to. And John Wesley Harding. Incredibly basic production of strange, mystical, religious stories. Oblique lyrics means you won't know exactly what it's all about, but the images painted are often unsettling.