First, avoid "highlights". I generally detest highlights, but I feel that in Wagner's case listening to so-called highlights would be particularly wrong.
I think that
Tannhaüser is a good introduction to Wagner in general. It's lovely music, standard length (i.e. three hours or so), and does not demand too much.
However, if your interest was piqued particularly by the compositional aspect I described, i.e. by Wagner's motif syntax, you definitely need to listen to the Ring (
Der Ring des Nibelugen), starting with
Das Rheingold, the introductory opera. Das Rheingold is short (about two hours), has a good, action-packed plot, and introduces the major motifs used in the Ring (Valhalla, the Ring, the Love-curse, the Spear, etc.)
An excellent way to economize while getting to really know the Ring is to get "The Ringdisc" -- an excellent multimedia offering by Media Cafe. It is a computer CD-ROM that includes the complete Ring in digital audio, in the performance is probably the most classic version out there: Georg Solti conducting the Vienna Philharmonic Orchestra in a studio recording; see
the Ring entry on Wikipedia (I recently added the recordings section) for details on that.
Anyhow, beside the music itself, the CD includes a Piano/Vocal score, an index of motifs, some images from past productions, and short write-ups on the characters and the concepts. The real value, though, is
running commentary, which the program displays alongside the score while the music plays. This commentary can point your attention to appearances of motifs, to subtle orchestral effects, etc. I found it an excellent "study guide" for the Ring.
As far as I remember, I bought it for about $80. Of course, being compressed digital audio, it's not quite CD quality sound, and if you want to experience Wagner proprely, you'd better get a good recording of Das Rheingold on CD (most performances fit on two CDs, so they should not be very expensive). I do recommend the Solti version, or Levine conducting at the Met.
You're quite welcome to borrow any of my Ring versions. Beside the Ringdisc, I have the Ring on CD performed by Solti and the VPO, Bohm at Bayreuth, and Furtwängler at La Scala (all mentioned in the Wikipedia page linked to above), as well as DVDs of Levine at the Met and parts of Goodall on CD and of Boulez on DVD. You are welcome to either come and pick them up (do!), or I could mail you one.