While I don't play it much nowadays, if there was to be only one variant available it should be seven card stud. In my opinion it is the most challenging because of it's complexity due to: more cards in play, five betting rounds, all of your cards are different than the ones your opponent(s) holds, each player has three unknown cards, to play well you have to also remember the cards that have been folded, and it increases the possibilities for effective bluffing. All in all it makes for a more interesting and exciting game
PS - Another reason is that I'm a very sentimental and have been playing this, known to my father as "Seven Toed Pete", for over fifty eight years.
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