Mendelssohn's String Octect, op. 20, any suggestion?

If you have the I Musici 1966 recording, it is excellent. The Academy SMF is also top shelf, but the recording I am familiar with is on Phillips (420-400-2), not Argo. Another good one is the Cleveland and Meliora Quartets (Telarc CD-80142)

Either the Academy of St Martin-in-the-Fields performance on Argo (ZRG 569 on vinyl) or the Vienna Octet performance on London (STS 15308 on vinyl) will offer you an excellent and very different alternative to the I Musici. Both performances are outstanding, yet each is also very different from the other.

Off the wall suggestion: There's a new multichannel DVD-A of the Octet on Tacet with the instruments in a ring around the listener. It's fascinating as the melodies zoom around the room.