It depends on how you define "best" and "worst." For example, while a game like video poker may offer the best odds when played perfectly, perfect strategy may not be one that the average player can implement. Additionally, the money-earning potential of a game can be impacted by the imposition of low limits or a slow rate of play. When it comes to gauging the worst, you can rate bets by their house edge alone or add in considerations of speed and minimum-bet size. The following looks at the question from the standpoint of a single bet only.
Worst Bet Under the single-bet criterion, keno is the worst. Though rare games surface with house advantages below 10% (currently El Cortez on certain tickets), the average edge is 25%-30% and the worst cards routinely exceed a 30% player disadvantage. What’s more, these are fixed percentages that don’t improve. The casino advantage on certain table-game side bets, such as Let It Ride and Caribbean Stud, may exceed that of keno, but they usually include progressive payoffs that improve their returns rapidly. Sports-betting parlay cards (especially "ties-win" varieties), can have holds up there with keno, but not all cards are this way. Keno gets the nod.
Best Bet The best bets for skilled players are found in blackjack and video poker. However, these games require study to master their strategies. Of the bets that anyone can make, the best is the banker bet in baccarat, which carries a low 1.06% house edge. Arguments will come in for craps’ pass and don’t pass with odds, but the line bet and odds bet are two separate (albeit linked) wagers.
The real determinant of the best and worst bets, however, should take into account the factors mentioned at the top of this answer. Jean Scott does this via an incredible chart on p. 33 of her book More Frugal Gambling. In ranking 40 bets in terms of their "dollar-per-hour cost to play," she identifies the true best and worst wagers. At the bottom of the list is dollar slots, at a cost of $54 per hour. At the top is sports betting, with a cost of just 25¢ per hour. And landing in the fourth-best position, at $2 per hour, is $1-per-game keno, which was named worst single bet above. This demonstrates the powerful effect that speed of play and amount wagered has on how much it costs to gamble.
Note: For an excellent reference of odds on all casino games, see the book Gambling 102 by Michael "The Wizard of Odds" Shackleford.
