There have been some might outstanding years by some very outstanding players, but the best performance was by Ty Cobb in 1911. I used as my source the phenomenally thorough Baseball Reference. The site is a baseball nerd/number cruncher/SABR metric devotees Paradise and has over 30 offensive categories. These include hit by pitch, sacrifice hits, etc. My much used 1984 Baseball Encyclopedia lists 13 categories.
I used 21 categories. Cobb led the American League in 19 and both leagues 18 of them. He came in second twice and only in Home Runs did he lag by winding up in 7th place, but second in the American League.
BOLD CAPS =LED MAJORS
Bold Letters= led American
No bold =did not lead.
Triples-24(The Major League Leader was with 25.
Homers-8(Came second to Frank Home Run Baker who hit 11.
On Base Pct.-.467(Came second to Shoeless Joe Jackson with .468
ON BASE+SLUGGING PCT- 1.088
EXTRA BASE HITS-79
Due to the interest of SABR metrics and other mathemtatical formulas designed to evaluate players from one era to another and to dig beyond the fundamental surface of the above mentioned categories, I added eight more. I will not get into too much detail but they are such as:WAR(Wins above replacement(player), RUNS CREATED, OFFENSIVE WINS and five others. Cobb led the Majors in all eight of them.
Cobb is one of the most maligned players due in large part to a bogus 'biography' by Al Stump, and refuted in Charles Leerhesen's book, A Terrible Beauty.
He is also arguably the best to ever the play the game. I don't think there's any argument that Cobb's 1911 was the best year of any player.