The state of Nevada shows detailed records for gaming stats including itemized casino win percentages: http://gaming.nv.gov/index.aspx?page=149

These were among the worst bets for casino gamblers from October 1, 2013 to September 30, 2014:

Sports Parlay Cards
Casino Win Percent: 37.65%

3-Card Poker
Casino Win Percent: 30.63%

Keno
Casino Win Percent: 25.79%

Let It Ride
Casino Win Percent: 24.44%

Pai Gow Poker
Casino Win Percent: 21.89%

I knew parlays and keno were bad but the others were kind of surprising.