If you are talking about per season, it's baseball.
This is all you need to see:
http://en.wikipedia.org/wiki/List_of_largest_sports_contracts
No other sport is giving out contracts like the above,
guaranteed. Baseball is the richest American sport in the world on a contract basis per season.
Kobe (highest contract in the NBA) doesn't even crack the top 20 richest contracts in sports, yet baseball holds 17 of the 20. Only Mayweather (tied for 8th, boxing), Kimi Raikkonen (14th, auto racing), and Messi (17th, soccer) crack the top 20 contracts that aren't baseball.
Rank Player Team Sport Length of contract Contract value (USD) Average per year (USD) Average per game7 (USD) Ref
01 Alex Rodriguez New York Yankees Baseball 10 years (20082017) $275,000,000 $27,500,000 $169,753.09 [1]
02 Alex Rodriguez3 Texas Rangers* Baseball 10 years (20012010) $252,000,000 $25,200,000 $155,555.56 [2]
03 Albert Pujols Los Angeles Angels of Anaheim Baseball 10 years (20122021) $240,000,000 $24,000,000 $148,481.06 [3]
04 Joey Votto Cincinnati Reds Baseball 10 years (20142023) $225,000,000 $22,500,000 $138,888.89 [4]
05 Prince Fielder Detroit Tigers Baseball 9 years (20122020) $214,000,000 $23,777,777 $146,776.40 [5]
06 Derek Jeter New York Yankees Baseball 10 years (20012010) $189,000,000 $18,900,000 $116,666.67 [6]
07 Joe Mauer Minnesota Twins Baseball 8 years (20112018) $184,000,000 $23,000,000 $141,975.31 [7]
08 (tie) Floyd Mayweather ShowTime Sports Boxing 2 years (20132015) $180,000,000+ $72,000,000+ $30,000,000+ [8]
08 (tie) Mark Teixeira New York Yankees Baseball 8 years (20092016) $180,000,000 $22,500,000 $138,888.89 [9]
09 CC Sabathia New York Yankees Baseball 7 years (20092015) $161,000,000 $23,000,000 $141,975.31 [10]
10 (tie) Matt Kemp Los Angeles Dodgers Baseball 8 years (20122019) $160,000,000 $20,000,000 $123,456.79 [11]
10 (tie) Manny Ramírez Boston Red Sox* Baseball 8 years (20012008) $160,000,000 $20,000,000 $123,456.79 [12]
12 Troy Tulowitzki Colorado Rockies Baseball 10 years (20112020) $157,750,000 $15,775,000 $97,376.54 [13]
13 Adrián González Boston Red Sox* Baseball 7 years (20122018) $154,000,000 $22,000,000 $135,802.67 [14]
14 Kimi Räikkönen Ferrari Auto racing 3 years (20072009) $153,000,000 $51,000,000 $2,942,307.69 [15]
15 Miguel Cabrera Detroit Tigers Baseball 8 years (20082015) $152,300,000 $19,037,500 $117,515.43 [16]
16 Todd Helton Colorado Rockies Baseball 11 years (20012011) $151,500,000 $13,772,727 $85,016.83 [17]
17 Lionel Andres Messi FC Barcelona Association football 8 years (20082016) $148,800,000 $18,600,000 $500,000.00 [18][19][20]
18 Cole Hamels Philadelphia Phillies Baseball 6 years (20132018) $144,000,000 $24,000,000 $148,148.15 [21]
19 Carl Crawford Boston Red Sox* Baseball 7 years (20112017) $142,000,000 $20,285,714 $125,220.46 [22]
20 Johan Santana New York Mets Baseball 6 years (20082013) $137,500,000 $22,916,667 $141,460.91 [23]
ESPN says baseball and boxing:
http://sports.espn.go.com/espn/news/story?id=6391391
Another indicator that baseball is the highest U.S. paid:
The World Series is the highest paid , per player, non-soccer event in the world ($377,000 per player on the Giants last year which was twice as much as the Ravens).
http://www.forbes.com/pictures/mme45hiih/4-world-series/
This is why baseball gets so much media attention. Also take into account the average NBA or NFL career vs MLB. It's not even a competition as to which sport an athlete will make the most guaranteed money due to contractual stability and career longevity.