I really can't think of a bad one.
That's because there aren't any bad ones. Their latest, Everything must go, didn't blow me away, but it's got it's moments.
I would recommend Citizen Steely Dan if you've got the cash.
As I see it, there are basically 4 eras of Steely Dan. There is the original era when there was the semblance of a band. Becker and Fagen were in charge from the get go, but there were other permenant members then. They made five albums, Can't Buy a Thrill, Countdown to Ecstasy, Pretzel Logic, Aja, and The Royal Scam. All are solid gold in my opinion. If I had to choose a favorite from this bunch it would probably be Pretzel Logic. The next era is when Becker and Fagan let the rest of the band go and worked exclusively with top flight studio musicians. Their sound became much more precise. They made two albums in this way. They are Aja, and Gaucho. If I had to choose between the two I would probably choose Aja, but they are both great. The third era is the 19 years they took off. During this time Becker retreated to Hawaii and became a producer. He's produced some albums of note, but I can't name them off the top of my head. The Offspring comes to mind for some strange odd reason, but I could be wrong about that. He also recorded a solo album sometime in the 90's, but I haven't heard it. He sings a song on the SD live album and he's not got a good voice. Fagan recorded two albums during this period, The Nightfly, and Kamakiriad. Both are very Steely Danesque. The Nightfly is one of my favorite albums. The fourth era is when Becker and Fagan reconvened in 1996 and began touring. I've seen them twice perform and they put on a good show. They recorded a live album sometime in the late 90's, which is alright, but it's basically exact reproductions of their studio cuts. They returned to the studio in 2000 and produced a latter day masterpiece with Two Against Nature. It's It's one of their best albums, I love it. I believe it won Album of the Year at the Grammys. Anyway, they released Everything Must Go last year and it's pretty good.
I hope this helps.
~Paul