According to the History of the Gun series, Browning considered his best work to be the Over and Under Shotgun.
I have several of his designs; the 1911, P-35, Winchester Model 12; Citori; BSS and many other products that employ his design innovations such as the lock up on the P-35. Of mine, I am overwhelmed with all of them but of all his designs I think the A-5, M-2, BAR and P-35 rise to the top.