The best play I can recall was at a locals' casino that had a promo two days a week. You got a scratch off ticket for every four of a kind. If you were playing .25 and above, you got one kind of scratcher---usually worth $5. .05 and .10 play got a $2 card. .01 play got $1.
You can see how the .01 prize was, on a percentage basis, much greater than the others. It amounted to 20 extra bets every roughly 424 hands. In other words, an almost 5% boost to EV. Even the worst games would be strongly positive.
This was more a curiosity than a viable play until I noticed that they had a bank of hundred play games, and some were .01 denoms. I found a full pay game, much to my surprise: 9/7 TDB, which returns 99.54%. The final piece of the puzzle was: would they pay multiple quads on the same hand? I found out that they would.
So here I had an effectively $5 a hand game that returned over +4%, and with very little variance. Hundred play goes more slowly than single line, but with the "Turbo" setting engaged, I managed 600 hands an hour. $1800 an hour at +4% is $72 an hour.
I sped things up by making sure that a slot attendant was always hovering nearby to give me my scratchers---I always tipped her. What blew my mind is that I was always alone on that bank--no one else seemed to know about this play. It lasted three months, until they didn't end the promo, but they removed the hundred play machines. By then, I was about $15,000 to the good.
Probably the best play ever was when Texas Station first opened. Double pay on all four of a kinds. You can imagine--after word got out, the place was mobbed. To give them credit, they stuck to it for five days of the originally announced week before they pulled the plug. I heard that the promo had cost them $26 million.
