For "all poker variants", much depends on which kind of poker is meant. Poker at a casino comes in two very different forms. In house games such as three card poker or casino hold'em, each player faces the dealer and a paytable. In room games such as hold'em or Omaha, players compete with each other and the casino takes a small cut of each pot. The hand rankings are shared by all of them, and the other rules vary between games.
Every poker variant ranks hands the same way, from a high card at the bottom through a pair, two pairs, three of a kind, a straight, a flush, a full house and four of a kind, to a straight flush at the top. Three card games are the exception in one detail: with fewer cards, a straight beats a flush. The ranking chart is printed in the rules of every game and worth keeping to hand.
Casino hold'em and Caribbean stud follow the same ante and raise pattern against the dealer. In casino hold'em, the player sees two cards and the first three shared cards, then folds or calls with a bet of twice the ante. In Caribbean stud, five cards are dealt to each side and the raise is also twice the ante. Both games require the dealer to qualify, and both pay bonuses on strong hands.
Yes, the best play in house games is fixed and printed in guides, such as playing any three card hand of queen, six, four or better.
Pair plus pays on the strength of the player's own three cards, whatever the dealer holds, and sits apart from the ante and play contest.
In three card poker the dealer needs queen high or better; without it the ante pays even money and the play bet is returned.
In room games such as hold'em, Omaha and stud, players compete with each other while the casino only hosts, so skill against opponents decides results over time.
After seeing two private cards and the first three shared ones, the player either folds or calls with a bet worth twice the ante.