The answer depends on at least three factors:
(1) Your skill, discipline and temperament
(2) The house rules of the game at the particular table where you are playing
(3) What you mean by "best"
(1) Your skill level is not particularly high or low and
you have the discipline and temperament to limit your bets to the best odds available
(2) Standard house rules
(3) By "best" you mean the lowest house advantage
The best game is craps making only line bets (pass, don't pass, come and don't come) and associated free odds bets. The next best is standard baccarat, making only bank and player bets, preferably on a table that only collects a 4% commission on banker bets.
If you are willing to boost your skill and discipline levels to the point that you can determine which house rules are better for the player and can count cards, the best is blackjack.
If you are willing and able to bring your skill level to the highest level, live poker is the best. The difficulty in reaching this skill level is extreme, and few players can reach it.
There are some video poker machines that have a very low house advantage, but you may have trouble finding such a machine in the average casino.
Roulette, mentioned in other answers, is not good choice unless you can find a table without a double zero and which has "la partage" rule or the "en prison" rule. These are rare in the United States.
There are times that a slot machine with progressive jackpots may give you a better chance of winning than table games, but this is a highly unusual situation and nearly impossible to accurately determine.
With the exception of free giveaway gifts or free play offered by casinos to lure you into the casino, you should not expect to actually come out ahead in a casino. The house always has an advantage. Even if the odds were completely even you would run out of money when you hit a losing streak but the casino would not run out of money when you hit a winning streak. Almost all players will give back some or all of their gains after a winning streak.