This question comes up frequently and it's difficult to answer, because many factors besides the rules and number of decks determine how "favorable" a game is. One primary factor is the betting limit. A game may have good rules, but if it'a a $100 minimum table, not many people will want to play. One game in town has excellent rules, but a max bet of $50; again, that will hobble serious blackjack players.
Other factors are the atmosphere of the casino (sawdust joint or carpet joint), comps you can earn, friendliness of the staff, etc.
But to take a stab at the "best" game, there are still a few lingering single-deckers with the traditional 3-2 payoff on naturals (6-5 is horrible and should be avoided). The El Cortez, Fiesta Rancho, Four Queens, Golden Nugget, Gold Spike, and Las Vegas Club have it.
In addition, you can find many good double-deck and 6-deck games that have a house edge of 0.2% or less. In particular, look for casinos that pay 3-2 on blackjack, stand on soft 17, and allow double after split. For 6-deck games, you'll also want to know if they have the surrender option available and if you can resplit aces.
The best way to save time finding the best games in Vegas is to get a copy of Stanford Wong's Current Blackjack News. It lists all the casinos in Vegas (and in the U.S.) with the number of tables, betting minimums and maximums, rules, number of decks, and other pertinent info. It's available at www.BJ21.com or by calling 702/579-7711.
