Oh man, I could go on and on. I am a huge Willie fan, and have quite a good chunk of his albums.
Two great early albums:
Phases and Stages; Shotgun Willie
These albums have many of his classics on them, along with many great songs many have never heard.
Stardust: This may have to be his prettiest album. The perfect album to turn the lights off too and sip your adult beverage of choice.
Willie and Family Live: This is probably the best party album from Willie. Many of the songs are done a little faster than the studio versions. It also gives you a peak of how great his live shows are.
Two recent albums: Songbird; It Always Will Be
I really enjoy both of these albums. Both are fairly raw with many tracks that have a nice bluesy feel.
Again, I love just about all of his work, but these albums will give a great look into what Willie has to offer.