Bartok is both the best and the worst because it is everything at once.
Summary
Bartok is a very simple game like Uno, where the winning player adds a rule. This simple core can fit almost every group size and mood.
Setup
You need a deck of cards per 5 players (rounded up). All players should be able to reach the middle of the table-ish.
Rules
Round Setup
The role of the dealer goes around the table. The dealer deals 5 cards to each player. 5 is a good starting number, but it can be increased if the game becomes faster. The remaining cards are placed in the middle of the table face down and one card is flipped over to form the base of the play pile. The player left of the dealer starts the round.
Round Turn
On your turn you can play a card if it matches the rank or the suit of the top card of the play pile. "Playing" a card means to put it on top of the play pile. If the player cannot play any card, or doesn't want to, then they have to draw a card from the face down pile. In either scenario, their turn is over and it is the next players turn. Whenever a player has one card left, they have to say "Bartok" out loud.
Player Mistakes
If a player makes a mistake they have to draw a punishment card . If a player played an invalid card on their turn, they pick it up and get to try again. If a player played out of turn they also pick it back up. In general players should do their best to "rewind" back from the mistake, the only evidence of the mistake being the extra punishment card.
Running out of Cards
If the face down pile runs out of cards and nobody can draw cards, the game enters sudden death! Whenever a player tries to draw a card, what happens instead is that each player discards a card, starting with the player left of the person that was trying to draw a card. If a player discards their last card, they win.
Round End
Once a player has no cards, they have won! That player has to make a new rule for the game. It is very important to redistribute wealth; If one player wins and more than one win ahead of the other players, they have to give away their right to make a rule. This ensures that all players create about the same number of rules.
Example Rules
A rule can be anything but an easy place to start is to add your favorite Uno rules, like reversing turn direction on 7's, skipping a player on 3's, or making players draw 2 cards on 2's. More complex examples could be to disallow asking questions, allowing players to cheat once per turn, playing with multiple play piles, allowing to play face down cards and let players "call you bluff", etc.
Meta Rules
Sometimes situations or rules are ambiguous and cannot be solved in an objective way. Sometimes a creative player will introduce a rule that nobody else wants to play. In these cases the underlying rule is democracy. Its not a rule unless most people accept it, its only a mistake if most people think it is.
Unexpected Rule Interactions
Another meta question is about interactions between rules. If two rules interact in an unexpected way, it is sometimes not clear what should happen. In that case, pause the game, and discuss what should happen in the situation. Prefer to give lead to the makers of the interacting rules, but ensure collective agreement.