Braxton plays and composes such a wide range of music that it's tough to find a starting point, many of his recordings are hard to get into even for someone like me who enjoys free improv. A lot of Braxton's output is on European labels which makes them expensive to buy in the USA; by now he must have at least 100 albums. I have about half a dozen, no two are much alike. The Mosaic box is a big and risky investment if you've never heard him before.
I agree with Passinwind,
Conference of the Birds by Dave Holland is a great one although it focuses on him as a player only. Obviously, plenty of great bass on that one
Braxton's only mainstream set I know of is Dave Brubeck's
All The Things We Are which also has Lee Konitz (note: Braxton is not on all the cuts).
You might also try
Paris Concert by
Circle (who were Chick Corea, Holland, Barry Altschul and Braxton). A nicely recorded live set from the 70s, all four contribute compositions so you will start to get a feel for Braxton's writing.
All three CDs are easy to find as they are on ECM and Atlantic.