The best thing I ever let go of was my 1965 Stratocaster. Sold it last year and made enough money to buy 3 newer guitars. (I don't follow the rule of disposing of one thing for every thing I take in.)
In a more audiophile context, the best thing I disposed of was probably my Magneplanar 3s, that I sold when I bought my Apogee Duettas. The Maggies were nice for smaller-scale acoustic music, but lacked the bass that the Duettas have in spades.

