Green’s Dictionary of Slang

first-rate, the best.

1915		Bulletin (Sydney) 2 Sept. 26/1: The night afore last one of the guns from the den up the alley whooshed me a bonzer on the mush with a duster, and I saw two snifter spangles.
1924		(con. WWI) A.G. Pretty Gloss. Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: snifter or snodger. Extraordinarily good or big.
1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 261: Snifter: Good. Satisfactory.
1941		A. Gurney Bluey & Curley 29 Mar. [synd. cartoon strip] He’s a beaut medium-fast, with a snifter off side leg break.