If you are talking vinyl ... try The Great Summit: Louis Armstrong and Duke Ellington. Two other excellent albums are Satch Plays Fats (Waller) and Louis Armstrong Plays W.C Handy.
The Armstrong/Ellington LPs are great performances and I play them regularly, but the sound quality is only okay. Better sonics and still great music will be found on:
Ella and Louis, Verve 4003 (Speakers Corner reissue)
Satchmo Plays King Oliver, Audio Fidelity ST-91058 (Classic Records reissue. For a real treat, get the 45 r.p.m. two-song sampler containing "St. James Infirmary")
Louis Armstrong meets Oscar Peterson, Verve VS 6062 (Speakers Corner reissue)
I love the Ella & Louis LPs and Satchmo Plays King Oliver but if its Louis' best music you are after you need to get the Hot Five and Hot Seven recordings from the beginning of his career. These are not audiophile fair, but they are musically astonishing. Columbia released these as LPs several years ago, but even if you can only find these recordings as CDs they are so essential that I would never let that stop me from getting them.